It's official; even Marvel Comics can't keep track of X-Men continuity. The X-Men comics are notorious for their incredible complex continuity, which is regularly retconned. Making matters worse, X-Men fans are very much continuity-savvy, able to pick up on the slightest errors. It's caused several past writers to openly complain, most notably Grant Morrison, who famously described the franchise's continuity as "septic like a toenail."
Enter Jonathan Hickman, the superstar comic book writer at the forefront of Marvel's X-Men relaunch. His House of X and Powers of X miniseries rewrote the X-Men timeline, with countless retcons implemented in order to fashion a brand new mutant status quo. It's proved to be a hit with readers, but it's probably more than a little challenging for other writers and editors to get their heads around what's changed and what's stayed the same.
Mark Waid's History of the Marvel Universe is a perfect example. It's a fantastic miniseries that presents a complete Marvel timeline, running from the creation of the universe through to its last days, when only Franklin Richards and Galactus are left. Significantly, the latest issue includes accounts of two attempted genocides of the mutant race. The first was seen in New X-Men #115, which told the horrific story of Genosha. Over sixteen million mutants had settled on the island nation, but they were wiped out in a devastating attack by rogue Sentinels.
The second was an event known as the Decimation, when the insane Scarlet Witch used her reality-manipulating powers to almost mutants from the face of Earth. According to History of the Marvel Universe #5, Scarlet Witch reduced the mutant population "from millions to mere hundreds."
The problem, however, is that X-Men retconned all of this just two months ago. In House of X #4, Jonathan Hickman presented an infographic showing the number of mutant deaths over the years. Seeking to emphasize the importance of Genosha, he revealed that there were only 17.5 million mutants living on Earth before the Sentinel attack. The Sentinels slaughtered over 16 million of them in one fell swoop, leaving just 986,618 left. The Scarlet Witch then further reduced the world's mutant population, de-powering all but 200 of them (give or take).
Of course, it's impossible to reconcile these two conflicting accounts. Did Scarlet Witch depower millions of mutants, or did she erase the powers of just under a million of them? Waid is intimately familiar with the X-Men franchise, and was responsible for several stories set in the post-Decimation timeline. As such, he's gone with the version of events he's familiar, and was probably blissfully unaware Jonathan Hickman had just rewritten history. It's all turned into a perfect illustration of just how crazy X-Men continuity really is.
History of the Marvel Universe #5 is on sale now from Marvel Comics.