The Official Guide's math questions are, generally speaking, not as difficult as they could be.
Any CAT, whether from a test prep company or GMAT Prep, is likely to show you (if you are doing reasonably well) questions that are harder than almost any of the ones in the OG. Having said that, the range in your scores is very large, and I would be nervous about assuming that any of them are "accurate." I wouldn't assume that your "true level" is either 21 or 35. It's probably somewhere in between. The best way to gauge where you really are is to take a GMAT Prep test, but unfortunately you can only do this twice without seeing repeat questions, so you need to weigh how much you want to know your level now vs. how much you will want the realistic practice later.

Hi Gmat boost. Thanks for getting back to me. I don't think my post was that clear so just to reiterate:

Diagnostic GMAT, Q28: (At this point it had been nearly 8 years since I did any kind of Algebra or Geometry, so wasn't surprised by my score. I probably blindly guessed on 20 to 30% of the problems.

1st Practice (Knewton) Q25: This was about 2 weeks after starting the online Knewton class so the drop hear doesn't make sense to me.

2nd Practice (Knewton) Q21: This is the score that doesn't make sense to me, since it was three weeks after starting the Knewton online course and at this point algebra concepts were starting to come back to me. However, at that point still had no review of geometry concepts so blindly guessed on all those problems. But still I find this to be a huge discrepancy between diagnostic.

3rd Practice (Manhattan) Q35: Looking at my first diagnostic and this score I'm actually not surprised by this score. This feels "more right" to me since I felt a lot more comfortable with algebraic concepts at this point. Going from a Q28 to Q35 in a month seems reasonable to me, since I was averaging about 3 hours of study a day.

I still believe the Q21 was a fluke since it was considerably lower than my diagnostic, when I blindly guessed on at least a third of the problems. My guess is that currently I'm in the lower 30's range but who knows! I will definitely take the GMAT prep practice. What I have notice however, is that the problems in Knewton's practice homework and on the CATS have been notably harder than the problems I saw on the Manhattan CAT or have seen in the OG. So was just curious about the discrepancy. Thanks again for responding.