Because (assuming you're only using WAN and LAN interfaces), every packet that goes "out" on the WAN interface will have first come "in" on the LAN interface (from m0n0wall's point of view).
I understand now, thanks. The point of view was what mislead me in the first place.
In any case, keep up the good work, m0n0wall is just great and I've learned to like it even more in the process of using it