Price difference between qpx api and ita matrix

Question

We have been using ITA matrix to see the lowest fare of flights. Recently we learned that there is an API for same and started checking Google QPX Express. But we noticed some variances example fares of some airlines does not appear on the API output while it's showing on the web. e.g. Qatar Airways (QR)

here is the sample flight

LOS-DXB (Nov 19,2015) DXB-LOS(Dec 22, 2015)

using https://qpx-express-demo.itasoftware.com/ & http://matrix.itasoftware.com/

the ita matrix displays Qatar airways. but the qpx does't.

why is this happening?