One subtest for a group cognitive test which my ds took at school indicated that he had attained the highest score possible but the percentile was recorded as simply 99th.

I�ve never heard of a 100th percentile either and I didn�t realize it was possible.

I had read somewhere, that on some tests, if 5 percent score perfectly, they may have randomly assigned scores of 95th - 99th percentile, while on another they would all be at the 99th percentile or all at the 95th percentile.