Nas was NOT on equal billing with AZ and Foxy. He was the superstar of the group. AZ’s Doe or Die came out in 95 and debuted at number 15 with one Gold single. Foxy dropped in 96 and debuted at number 7 and went Plat. Nas was coming off of It Was Written which debuted at number 1 and stayed there FOUR weeks in a row, went double platinum and had two platinum singles.
Yet, Nas was still considered the same level as AZ. It doesn't matter what he sold, we didn't care back then, cause no one ever mentioned them as not equals, and by fall '97 when that album dropped, Foxy was bigger than all of them...whatever Nas had done in '96 was no longer shining as bright. He was doing Will Smith ish, and '97 Escobar had already dropped which most people weren't feeling. Nature was the only newcomer. It was pegged a "supergroup." It might have been the first attempt at a supergroup. It wasn't Nas & his underlings. It wasn't called Nas presents The Firm, the official title was The Firm - The Album, with (Nas, Foxy, AZ and Nature Present). If anything, you can argue that anticipation for The Firm is what kept Nas somewhat in the mix in '97.
Similar example - Meth's Tical in '94 outsold Rae '95 & Ghost '96. Them dudes were still considered the same level. Nas & Mobb - Nas was selling more, but Nas & P were considered on the same level.