It also depends on map, I believe
If you're good at e.g.: goldrush

you need to keep improving to earn better rate.
If your average on grush is, let's say: 18 ppm
and you have a good k/d ratio and your ppm session is marked : 19 ppm or 20 ppm

you won't gain much rate (or even lose), because you're already pretty good at that particular map.
I'm not really sure how the TB rating system works, because it's calculating many things -like objective play