Sorry, cant give a full reply have to run out. For the old tests, there are percentiles. They do not increase in difficulty with the test number. However, each test has a percentile rating based on the answers by previous GMAT Club members.
The new tests take the weights of each individual question to built out a test and present the score based on the individual question levels.
One "issue" is with scoring. The GMAT Club audience is very good in quant so the percentiles that you get with the old tests are quite low even if you hit quite a few questions right. Don't get discouraged - thats based on a high number of Q50 scorers.
Remnants of the same issue still exist in the new tests but we should be able to get rid of that and make them pretty balanced with the Real GMAT in the next few months (we need people to take the tests; and the more the better).
Hope this helps.
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