Because when you park your ass in the car on the left side and it loads the suspension, the camber evens out.

Since I can adjust the camber on the front end of my car, the shop I use will grab a guy about my size and sit him in the driver's seat while they are taking readings. When they get the car set up like it needs to be, they hand me a print out without the guy in the seat.

I watch them do the alignment, so I don't worry about it though.