I haven't taken AAMC 6 yet..but I will tell you I did better on the later ones (8 & 9) than I did on (4 & 5).
If its been a while since you've taken tests...and 6 is your more recent one..it could be because you actually know the material better and have practiced enough to improve your score. From the size of your jump ( about 7 points )..even if 6 is on the easier end of tests (which I've heard it is)...you definitely improved your score by at least a few points. So take this success in a good way
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Another reason might be...different tests have different topics on them..and maybe you got topics you were really good at on 6 that you didn't get on the previous tests. The best way to find out if you've actually improved and find out if your score for AAMC 6 isn't a fluke..is to take another test..take 7 and see if you do almost as good as 6. If you do, you know you improved
Hope I helped.