Why does prime 95 sometimes do 6 or 7 or 8 or 9 tests?

i got no idea but i would assume that the more CPU power you add (the higher your overclock), the more tests it would run because it takes more CPU usage to get to max load... That probably makes no sense.  

No I'm not sure that makes sense either...

If you up your clock your just going to do the tests faster but you are still doing just as many iterations of each test! maybe there is also a time element as well as number of iterations to make sure the tests run for a while so that the processor reaches max temp perhaps...

You can manually set the amount of iterations it runs of each data size (8K, 16K, etc). By default, it is set to do 15 iterations of each size on the small FFT test, I'm not sure about the large FFT test and Blend test, though, so depending on which test you run (or make a custom test), it will test different amounts.

Keep in mind that it is not technically meant for testing for instabilities, it's just looking for Mersenne Prime numbers, testing different data sets (and runs at 100% on each core   )