http://www.theeroticreview.com/discussion-boards/ter-general-board-12/x-out-of-y-scoring-is-a-10-out-of-10-idea-1009688 and many other threads on General and S&P.  
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Hextyn describes another example that shows how X-out-of-Y scoring is a 10-out-of-10 idea.
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But first, a recap of the Scoring Rules (from https://www.theeroticreview.com/info_policies/reviewFAQ.asp?#B ).

I don't really get the obsession with the 10 score. So what, if someone is really into anal or 3-somes it's clearly within the structure that girls the offer one or both of those options but refuse to DFK or perhaps BBBJ won't have 10 for their performance scores.

that a ten performance score should reflect, not WHAT the provider performs, but HOW the provider performs.

As a provider that 8/10 sessions I would qualify for a 10 in performance, I agree with the OP; it's ridiculous that some of my provider friends that I think CRUSH me can never receive a 10 because they don't offer threesomes nor anal.

X-out-of-Y is sort of 3 scores. I also think that X-out-of-Y would be easier to program and also easier to apply retroactively. Looks, Performance, Experience requires an extra new data field to be displayed. And I'm nearly certain that reviewers will not agree on what Performance and Experience are supposed to be.
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Nomenclature problem: WHICH is experience and WHICH is performance? I think you and I are opposite in how we would use the terms. I think that "Experience" is the list of "experiments" (DFK, BBBJ, ...). Performance is how well each experiment was performed. (I'm thinking back to college science lab! And the cute TA!)
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I can see using "Performance" for the list of experiments that were performed. But I don't see "experience" describing how well the experiment was performed. In future discussions, if any, let's make sure that we are specific in what we are referring to with each term.
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(1) Looks is separate, as now.
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(2) Y is the max allowed for that "Experience" = that session. No anal, no bi, no more than one guy, no DFK, blah, blah, blah ... The max "experience" score is a 7. Throw in anal and the experience is now an 8. Add DFK, 9. ...
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(3) X is the Performance score for how well each "experiment" was performed, although I think that most reviewers don't rate individual sub-routine. ("Well, the BJ was a little too toothy but the ball licking sort of made up for it. The DFK was only 3/4 of the way in, so I rate that a 7.5. And ...") I think that most guys just react to the composite whole. "That was a 10 Performance!!!"
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The TER Reviewers are already supposed to check that the Performance score (X) does not exceed the allowed maximum (which would become the Y). In other words, TER is already checking the Y values, just not recording them anywhere. Now, while checking a review, TER checkers would enter the Y (max allowed score) into a data field (that computes with the X field to display X-out-of-Y).  
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If the reviewer gave a Performance score of 10 but the max allowed is a 7, 8, or 9, the automatically computed X-out-of-Y is a 7-out-of-7, 8-out-of-8, or 9-out-of-9. Likewise, when the Reviewer's X exceeds the TER Y, it maxes out the score:
Original Performance X --> revised X/Y
10 --> 10/Y becomes 7/7, 8/8, 9/9, or 10/10
I'm not sure how to treat lower Performance scores on lesser Ys.  
MAYBE:  
9 --> 9/Y becomes 7/7, 8/8, 9/9  
8 --> 8/Y becomes 7/7, 8/8
7 --> 7/Y becomes 7/7  
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but MAYBE subtract the diff between X and 10:
9 --> 6/7, 7/8, 8/9
8 --> 5/7, 6/8, 7/9
7 --> 4/7, 5/8, 6/9
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In summary, I think that X-out-of-Y is an easier way to implement a 3 score system.

The highest score possible for a paralyzed person is a 7. Unfortunately because of his disabilities he has no movement from the waist down & is in a wheelchair. I got a review that did not help my score it hurts. I don’t think it’s fair & either did my client who wrote the review. Many providers won’t even cater to disabled people. If the person is incapable why should it damage the providers score? In my opinion I should get bonus points. Same goes for those who only want BDSM.