When creating a level for quake you often use six sided primitives to make walls. When you compile the level, the BSP tool strips out any polys that are outside of the level and won''t be seen. So while you see six sides in the editor, the game ends up using only one.

Another example would be a six sided pillar that touches both the cieling and floor. The four sides that make up the wall of the pillar are visible and would be kept, while the top and bottom will never be seen and are thrown out.

This is the reason why your levels in quake need to be sealed and have no leaks. Otherwise the BSP tool (Actually, I think it may be the VIS tool that does this), will be unable to determine what is inside and what is out.