Private or obscure ones I guess.

Private and obscure benchmarks are very often gamed by the benchmarkers. It's very difficult to design a fair benchmark (e.g chrome can be optimized to load Gmail for obvious reasons. maybe we should choose a more fair website when comparing browsers? but which? how can we know that neither browser has optimizations specific for page X?). Obscure benchmarks are useless because we don't know if they measure the same thing. Private benchmarks are definitely fun but only useful to the author.

If a benchmark is well established you can be sure everyone is trying to game it.