Currently the failed test rule and the passed test rule are not symmetric.
The failed test rule only gives points only for new failed test and the passed test rule only for new passed tests.
I.e. skipped test are lost along the way.
Build n : SUCCESS, 610 passing tests
Build n+1: UNSTABLE, 71 passing, 67 failed, 2963 skipped tests (TestNG has the strange behaviour that when the init of some test classes fails, the skipped count sky-rockets)
Build n+2: SUCCESS, 610 passing tests
Build n+1 gives 71 minus points
Build n+2 gives 539 plus points
So, by breaking the build and fixing it again you can 'unrightfully' get lots of points