Perhaps your, er, 'output' is related to what's going through your mind at the time. ;-)
When you're on the job with the missus, your probably as concerned about whether she's getting off as you are about having fun yourself.
But when you're on your own, the old grey matter can concentrate on processing your porn or whatever images are flashing through your mind... Self gratification indeed!
Perhaps you should run some tests -- see what happens when you use differnet material for, er, 'inspiration'!