It looks like levels 2, 3, and 4 just use the limit that has been set from a previous level. They don't actually set their own limit of 5 trees.
Level 1 sets the limit to 5 (which is the default when starting the game).
Level 5 sets the limit to 3.
Level 6 sets the limit to 4.
If I play one of those levels, then switch to 2, 3, or 4, the limit stays at whatever it was at from the previous level.
I'm on 64 bit vista, if that makes any difference.