I have several of their background buildings and 2 more yet to be built. All of them so far included the same amount of doors, window frames and glass as the full version of the same basic kit. So other than shorter side walls and not including the back walls and related full roof pieces mine have been complete with a lot left over. I use those extras for kitbashing projects here and there. My latest is the Parkview Terrace Apartments background kit which I got at the LHS a few weeks ago with his weekly order from Walthers and it is not missing any trim parts.
Just a guess but maybe they are trying to hold costs down by not including anything other than the exact amount of parts needed to build the background kit and at some point that plan got messed up.
Lobuc Valley RR
"The Hobo Route"