floor overhang

Hi All,
I asked this question once before but still am not clear as to why some builders leave a gap between the outside of the steel chassis perimeter and the plywood sidewalls. I know that lots of builders leave a gap, but am not sure why some do and some don't. In other words, why is the plywood floor often cut a little bit wider than the steel chassis frame? Would I run into any problems if I let my plywood overhang the sides of my chassis, butting tight against it? Is the gap left to allow for any slight inconsistencies in the frame?
Sorry if I'm repeating myself here.
Sometimes I need to hear information more than once or in a different way before it sinks into my teardrop-builder-overloaded brain.
Thanks for any help.
Kris
I asked this question once before but still am not clear as to why some builders leave a gap between the outside of the steel chassis perimeter and the plywood sidewalls. I know that lots of builders leave a gap, but am not sure why some do and some don't. In other words, why is the plywood floor often cut a little bit wider than the steel chassis frame? Would I run into any problems if I let my plywood overhang the sides of my chassis, butting tight against it? Is the gap left to allow for any slight inconsistencies in the frame?

Sorry if I'm repeating myself here.

Thanks for any help.
Kris