Primegrid is a special case. PPS Sieve is awarding more credits than other subprojects because you can't actually discover new primes when running PPS Sieve (you are doing some sort of "preparation work", check here for more details).

When running other GPU Primegrid subprojects (GFN), you can actually discover new prime-numbers pretty quickly and you get some recognition for that, so Primegrid admins decided that's already a big enough incentive and reduced the credits awarded for GFN (while increasing them for PPS Sieve).

The point is, any "rewards calculator" can be quite tricky to implement, since it will require in-depth knowledge of every BOINC project and they are bound to get more complex with time. In this case, it wouldn't be enough to point users to Primegrid, they would crunch GFN, earn mediocre credits and get disappointed. They must be pointed specifically to PPS Sieve, if you are promising big rewards to them.

P.S. I am a long-time Primegrid cruncher, got all this info by following their message boards for years, it isn't readily available anywhere else, as far as I know. BOINC admins can fine-tune credit reward system as they see fit, for every project and subproject. Perhaps WUProp can provide necessary correlation.