Heartless only spawning when walking through rooms in certain order?

The title more or less explains my question. Ive noticed while farming materials for list completion in KH2FM that sometimes additional heartless will only spawn if I go in a section exit from a different path and walk back in. Even if I full clear the room theres still times (depending on location the number of heartless will vary) where I will clear the room leave from a different exit and come back to find another 1-3 heartless will appear. Can someone explain this?

Thanks