This is because it doesn't take a tick to move to the next room, it's instantaneous, no commit is needed.
First: All creep movement resolves down to "creep.move(dir)" where dir is a direction constant, 1 to 8. 1 in due north, 8 is north west.4 is due south.
What's actually happening is when you register the intent "creep.move(4)" to move from tile 48 to tile 49 (The exit tile.) in the room above, you get moved all the way to tile 0 in the room below.
If the creep is talking, it will say what was registered to say in the previous tick, back when it was moving from tile 48.
If you're using "creep.moveTo()" or one of the stock path follower routines, the "lost tile" will be accounted for when crossing room boundaries, otherwise you have to manage it yourself.
I hope that makes sense. Room edge crossing can be a little hard to wrap your brain around, but there is logical sense behind it. That intent, where you leave the room, cannot fail as all you would have to do to block a room is jam creeps at the border.
