The combination of the backlog and the profiteering incentivised people to use test slots when they weren't test-ready because people were reluctant to lose the test slots that they'd prebooked. So they sat the tests, failed them and rebooked, instead of letting anyone else have the slot.
Hmm, I'm not sure I've explained that well. Let me try again.
So you're a teenage learner driver near a test centre that is booked 22 weeks in advance. You're not ready now, but your instructor and family are confident you'll be ready in about four to five months, so you book in advance. Perhaps by getting up early, or perhaps by spending money on one of those third-party sites. Point is, you have your test slot.
The date comes round, and you're not quite ready after all. But if you have a good day, you might scrape a pass! The backlog at the test centre has increased and it is now going to be 24 weeks before you can test, if you cancel this week's slot, and you're not going to get your money back from that website.
So what do you do? Especially if you're a teenager to whom six months' time feels like six years?
Well what thousands did/do, was sit the test and fail, and then rejoin the queue. Or they cancelled at the very last minute, which meant no-one who was more ready to pass had a chance to grab the slot instead.