I knew someone who had that policy and she was a legitimate and very talented Mistress. This was many years ago, early-ish in both our careers, when we got most of our business off Backpage and deposits weren't nearly as much of a thing. She lived close to the dungeon so same-day sessions weren't much of a hassle and she said advance bookers tended to be flakes. At the time she had a point -- I'd noticed the same thing, and most requests I got back then were same-day anyway so turning down advance bookings wouldn't have made much of a dent.

My friend has since changed her policy, and the business has changed a lot. That policy makes much less sense now than it did then, but as Domina M said she might just have an unpredictable schedule. If she doesn't require deposits and gets a lot of wankers that could also have something to do with it.