Johnsnow, I believe you’re over-thinking it!
The girls have a complicated job which ultimately is much more about satisfying you than themselves.
How often do they legitimately O? Nowhere near as often as mongers believe, or suspend disbelief to believe.
The best sex with SWs feels like with a girlfriend, but it’s not - obviously. So she’s almost certainly not apologizing for not having an orgasm, she’s apologizing for not synchronizing her performance with yours: something that can be corrected if truly necessary, in the next meeting.