Well sure, all bikes are different. As far as your 91 kdx, the bike is heavier, and the clutch is probably worn slap out. More weight on the rear tire and a clutch that doesn't grab as well may assist with pushing the bike in gear with the clutch pulled in. As far as "all the bikes you've ever been around", I know nothing about them. I can reiterate what I said in my first post, but I don't see the point in doing so. You know, that whole beating a dead horse thing kinda gets old.
If the clutch works when the bike is running, what difference does it make? Do you really need to be able to push your bike when the engine is off, and it's in gear? Just put it in neutral! That's just how it is. Stop fretting and go ride.
J.