They are not always lower. There is one in the top 10. I have seen some that are above 9. But on average, sure they are lower. Some may be cases like what you say. But a lot of it is something that applies to them, but not cis women.
The word of the day is versitile. So many say they are, but they really are not. This will lead to some disappointment, and she may look great, but not be what a client was looking for in his experience. If you want to be topped, and she is not doing it well, or in some cases will even say "I was hoping to bottom" or "I can't get it up today", you will be disappointmed no matter how hot she is. Even if you want to top her, you may want a stiff one to suck on before you get there. And so many times she can't get it up which you would expect she could if she was really versitile.
While I understand saying versitile when you are not may bring in more business, it will also bring more disappointment if you are really not. The ones I have seen that are rated high are honest. I have seen true bottoms who say that's what they are upfront. I know what to expect with them. I have seen true versitile ladies as well who are rated high. Now, I have not see a top, because that's not my thing, but the one provider in the top 10 on TER, well she is a top and she says so upfront. Honesty in this area goes a long way.
Adding to this, as someone who sees both CIS and Trans, I have found more Trans who don't really want to kiss and who do CBJ and not BBBJ. These will also hurt there reviews. Again, doesn't matter how good they look. These things should be upfront if you don't want them to hurt your reviews.
All that said, I'm sure your points have some validity too. Some guys may be experimenting and not really know what and then not review a Trans provider well because of their own unreasonable expectations. But it's not all on these type of clients.