Why Datepart function rounding the millisecond portion of datetime column

Question

I have a SQL Server table that has a datetime column. Its default value is provided by the GETDATE() function.

I want to select the millisecond portion of the value by using DATEPART() function. Interestingly, it is rounding the millisecond portion like 637 to 636. There is a problem.

What is causing the problem or how can I solve it? By using convert function I can exactly handle the millisecond portion but why is the DATEPART function is rounding I couldn't find.

Screen shot of the table query result:

The query

SELECT 
    [ID],
    DATETIME_COLUMN,
    DATEPART(ms, DATETIME_COLUMN) MS_PART_W_DATEPART,
    RIGHT(CONVERT(VARCHAR,DATETIME_COLUMN,27),3) MS_PART_VARCHAR      
FROM 
    DATETIME_TABLE

You can test with the script and data set.

Table script:

CREATE TABLE [dbo].[DATETIME_TABLE]
(
    [ID] [int] NOT NULL,
    [DATETIME_COLUMN] [datetime] NULL   
) ON [PRIMARY]

Data set:

INSERT INTO [dbo].[DATETIME_TABLE]
([ID], [DATETIME_COLUMN])
VALUES
(613577,    '2024-01-01 09:52:06.637'),
(613578,    '2024-01-01 09:52:39.050'),
(613579,    '2024-01-01 09:53:04.557'),
(613580,    '2024-01-01 09:53:36.200'),
(613581,    '2024-01-01 09:53:49.307'),
(613582,    '2024-01-01 09:53:55.813'),
(613583,    '2024-01-01 09:54:01.823'),
(613584,    '2024-01-01 09:54:45.277'),
(613585,    '2024-01-01 09:55:03.917'),
(613586,    '2024-01-01 09:55:27.340'),
(613587,    '2024-01-01 09:56:15.700'),
(613588,    '2024-01-01 09:57:11.493'),
(613589,    '2024-01-01 09:58:10.947'),
(613590,    '2024-01-01 09:58:52.040'),
(613591,    '2024-01-01 10:00:13.360'),
(613592,    '2024-01-01 10:00:59.837'),
(613593,    '2024-01-01 10:01:24.680'),
(613594,    '2024-01-01 10:01:54.480'),
(613595,    '2024-01-01 10:03:37.580'),
(613596,    '2024-01-01 10:04:48.683'),
(613597,    '2024-01-01 10:05:51.057'),
(613598,    '2024-01-01 10:08:00.070'),
(613599,    '2024-01-01 10:09:00.627'),
(613600,    '2024-01-01 10:10:30.693'),
(613601,    '2024-01-01 10:11:23.090'),
(613602,    '2024-01-01 10:12:00.417'),
(613603,    '2024-01-01 10:12:30.977'),
(613604,    '2024-01-01 10:14:46.230'),
(613605,    '2024-01-01 10:15:22.373'),
(613606,    '2024-01-01 10:16:07.153'),
(613607,    '2024-01-01 10:16:44.523'),
(613608,    '2024-01-01 10:17:27.480'),
(613610,    '2024-01-01 10:50:58.017'),
(613611,    '2024-01-01 10:51:20.437'),
(613612,    '2024-01-01 10:51:34.000'),
(613613,    '2024-01-01 10:51:57.530'),
(613614,    '2024-01-01 10:52:15.593'),
(613615,    '2024-01-01 10:53:49.623'),
(613616,    '2024-01-01 10:54:39.420'),
(613617,    '2024-01-01 10:55:10.610'),
(613618,    '2024-01-01 10:55:30.093'),
(613619,    '2024-01-01 10:55:54.220'),
(613620,    '2024-01-01 10:56:08.860'),
(613621,    '2024-01-01 10:57:00.830'),
(613622,    '2024-01-01 10:57:22.907'),
(613623,    '2024-01-01 10:57:35.447'),
(613624,    '2024-01-01 10:58:09.763'),
(613625,    '2024-01-01 10:58:42.780'),
(613626,    '2024-01-01 10:59:00.500'),
(613627,    '2024-01-01 10:59:55.590'),
(613628,    '2024-01-01 11:00:54.203'),
(613629,    '2024-01-01 11:01:31.047'),
(613630,    '2024-01-01 11:01:35.657'),
(613631,    '2024-01-01 11:01:38.483'),
(613632,    '2024-01-01 11:01:41.713'),
(613633,    '2024-01-01 11:01:44.360'),
(613634,    '2024-01-01 11:01:47.410'),
(613635,    '2024-01-01 11:01:54.907'),
(613636,    '2024-01-01 11:01:58.360'),
(613637,    '2024-01-01 11:40:37.320'),
(613638,    '2024-01-01 11:41:40.877'),
(613639,    '2024-01-01 11:42:01.297'),
(613640,    '2024-01-01 11:42:29.360'),
(613641,    '2024-01-01 11:42:51.533'),
(613642,    '2024-01-01 11:43:10.940'),
(613643,    '2024-01-01 11:43:50.097'),
(613644,    '2024-01-01 11:44:26.033'),
(613645,    '2024-01-01 11:44:42.317'),
(613646,    '2024-01-01 11:45:18.250'),
(613647,    '2024-01-01 11:45:45.597'),
(613648,    '2024-01-01 11:45:54.957'),
(613649,    '2024-01-01 11:46:11.287'),
(613650,    '2024-01-01 11:46:36.733'),
(613651,    '2024-01-01 11:46:57.487'),
(613652,    '2024-01-01 11:47:34.220'),
(613653,    '2024-01-01 11:50:25.503'),
(613654,    '2024-01-01 11:51:03.487'),
(613655,    '2024-01-01 11:51:13.737'),
(613656,    '2024-01-01 11:51:28.120'),
(613657,    '2024-01-01 11:51:44.520'),
(613658,    '2024-01-01 11:51:47.980'),
(613659,    '2024-01-01 11:51:56.707'),
(613660,    '2024-01-01 11:52:16.320'),
(613661,    '2024-01-01 11:52:26.113'),
(613662,    '2024-01-01 11:52:29.987'),
(613663,    '2024-01-01 11:52:32.940'),
(613664,    '2024-01-01 11:52:36.003'),
(613665,    '2024-01-01 11:52:43.080'),
(613666,    '2024-01-01 11:52:55.503'),
(613667,    '2024-01-01 11:53:29.017'),
(613668,    '2024-01-01 11:53:55.403'),
(613669,    '2024-01-01 11:58:16.183'),
(613670,    '2024-01-01 11:58:34.097'),
(613671,    '2024-01-01 12:02:31.927'),
(613672,    '2024-01-01 12:03:25.807'),
(613673,    '2024-01-01 12:08:14.473'),
(613674,    '2024-01-01 12:08:17.620'),
(613675,    '2024-01-01 12:08:45.040'),
(613676,    '2024-01-01 12:09:07.623'),
(613677,    '2024-01-01 12:09:13.787')

I tried to solve the problem on my own experience, search the net. Then decide to ask the question.

Result

After reading comments. I added a datetime2 column into the table and update its values from the datetime column.

.7 actually recorded as .66667