[Mivereous]

Might have something to do with Towns and Interiors (non-house) having to share large Z layers, as well, whereas housing just has to share with whatever other houses are active at the time, or otherwise nothing. Larger Z layers would cause more usage, as they're rendered even if they're off the visible screen. That would explain why going into a house (a Z layer where maybe 10 houses are allowed, only maybe 4 in use?) would cause less usage.