Compiler choose primary unique index - PROGRESS 4GL

Question

From below query I have given all the components for index vdmod_idx1. But when I checked XREF, I can see the compiler choose vdlist since it is primary unique. May I know why it is behaving like this. I would like to know what's happening here

And moreover, if I use index vdmod_idx1 in my query to then only it picks vdmod_idx1

for last vdhist no-lock
    where vdhist.vddomain  = dom
    and vdclassification   = 1
    and vdhist.vdlist      = prlt
    and vdhist.vdprod_ln   = prodLn
    and vdhist.vdcurr      = curr
    and vdhist.vdpart      = part
    and vdhist.vdum        = tt_um 
    and vdhist.vdstart     = date(tt_start)
    and vdmod_date > 01/01/2021
    and vdmod_time > 45556:
end. 

Index used

pu    vdlist 10 + vddomain
                + vdclassification
                + vdlist
                + vdprod_ln
                + vdcurr
                + vdpart
                + vdum
                + vdstart
                + vdcrt_date
                + vdlocal_rsn

vdmod_idx1      10 + vddomain
                   + vdclassification
                   + vdlist
                   + vdprod_ln
                   + vdcurr
                   + vdpart
                   + vdum
                   + vdstart
                   + vdmod_date
                   + vdmod_time