Well ... are you checking based on the same keyword? That's the first question. They both use 27 data points and I do think there's quite a lot of overlap, if not 100% ... so they should be fairly similar in terms of what's missing/what you should fix.
I just don't think they're graded the same way with the weights. As an example:
If I check my site http://mattantonino.com/ in the Page Optimization tab for my name, I see a score of 79 because I have addressed 22/27 factors.
Using the on-page grader I have also addressed 22/27 factors but ... I get an "F" for optimizing this keyphrase.
But the grader rates the importance of each factor - Critical! My title doesn't include my name. And two of the factors are "high importance." The grader says that as well - "We weight the items graded by their importance level, which we determine by our industry experience and correlation research."
I think the Page Optimization is not weighted the same way - it's more of "how many boxes did you check."
As long as your SEO is done fairly well, the on-page grader AND the Page Optimization should only be used as a guide. I used the above example to illustrate the point as I'm currently ranking #2 behind my well-optimized LinkedIn profile for my name. Yes, if I put my name in the title I would shoot to #1. But #2 and the page got an F on the grader.
I chose the #1 ranking site for "SEO" here locally and it receives a "B" grade with the keyword used 193 times. The sites ranked #2 & 3 get an A and the one ranked 90th gets an A as well.