They might have programed in the various models for the XY protags but only the defaults for ORAS ones and not have customisation for those.
Besides, do the oras models exist in XY, so maybe they will just show up as defaults there too.
Might I site Gen 4 as an example against this. If you connect to DP through HS, the DP person sees their local sprite of the appropriate gender, while the HS player sees the DP sprite. It could easily be the same thing for XY to AO.