If you have the pyramids (+2 GE points) and two hired scientists (+3 GS points each), then the pyramids are contributing 25% of the points, but you'll have a 33% chance of landing a GE (because one third of your sources is an engineer source).
This is why you get more Great Artists from your GP farm than you would like in a non cultural game: even though the National Epic is only contributing one point per turn, it contaminates the pool just as strongly as hiring an artist does.
It may not always have been that way, but that's the current state of affairs in Warlords 2.08 and Vanilla 1.61