I know it might be surpising that Makaveli didn't seem to sell as much even though it is highly regarded as a masterpiece is you have to understand Makaveli was a lot darker and not as commercial as the others(although it is false it was meant to be a straight-underground album).
Another thing is that for every album sold by GH or AEOM counts as 2 since they are double discs, so technically there are basically the same amount of peolpe buying the albums, but it is a little harder to sell a double album since it costs more, especailly to casual fans who just want to bump the singles, so in my mind, that is very impressive that Pac was able to do that.