It's not the most compelling complaint in the world, but what could the airline's rationale have been for putting the last minute solo non-status lady in occupied 11C instead of in empty 11B?

I can see no valid operational reason for doing that, except that this lady asked nicely for an aisle seat and the gate agent was sharp enough to notice a couple on the same reservation or with same name trying the split seats/empty middle seat gambit and she said "Oh screw them I'm giving this lady the aisle seat."