Jay-Z was just more consistent with his releases and maintained longevity.
But he didn't sell 3-4 million albums every album he was a steady seller though. Maybe with streaming now factored in he's do re-certification.
5,299,000, "Vol. 2...Hard Knock Life"
3,345,000, "Black Album"
3,093,000, "Vol. 3...Life and Times of S. Carter"
2,608,000, "The Blueprint"
2,521,000, "Dynasty Roc La Familia (2000 -)"
2,069,000, "The Blueprint 2: The Gift and the Curse"
1,934,000, "Collision Course" (with Linkin Park)
1,514,000, "Reasonable Doubt"
1,510,000, "Kingdom Come"
1,412,000, "In My Lifetime, Vol. 1"
1,131,000, "American Gangster"
Even though Nelly fell out commercially, he still rounds out in the Top 5 Best Selling Rappers list.