Hesketh Pritchard shows a drawing in his book which illustrates the advantages and disadvantages of each type. I would suspect that at first there was little official understanding of reticules at all, and crosshairs were probably supplied by Winchester by default. Later on perhaps, the advantages of the post probably filtered through the C of C and those were ordered instead/as well?
Purely speculation on my part, and those who have military marked WA5/B4 scopes can perhaps chip in on what reticules they have.
It comes down to visibility versus target obscuration. What was really needed was a vertical post from the top of the FoV down, as some of the later
German scopes had. However, the "good enough" mentality was almost as prevalent then as later.
The reticule in the Winchester scopes is just soldered to a thin sheet brass ring which was designed to be easy to replace, so it would be possible that reticules were replaced or changed post-war as well.