It is my impression that the majority of girls working SL do one-a-night (..."my impression"..."the majority"...). This is not high season so there are going to be some girls some nights who end up with no customers and a negative ledger for the day.
Sure, there are ZB girls who make nothing or next to it on some days but Karin is not one of them. She is likely to have a more stable income at ZB but, in the long run, she could probably make better money at SL, if she hung long enough to establish a clientele.
Two other issues: Intimacy and security. The SL, go to his room/apt for 1-3 hours, is a lot more intimate than the assembly line at ZB. Much easier for a girl to stay detached and look at it as just a job on the ol' job site. It is also much more secure since all her friends and staff are very near by.