Do we officially know why this was a thing? It was so strange and out of place.
To slow max level number inflation.
I think going into Cata they were considering expanding annually, instead of biannually. That didn’t happen, so we were stuck with 5 levels for 2 years, then 5 more in MOP presumably to get things back to an even keel (85-90). Ever since then it’s been 10 as per normal.
Believe me! In Cata, each level bar took FOREVER!
They didn’t want us to get to lvl 100 too quickly and reach crazy numbers like 110 and 120. But after Cata Blizz realized that odd numbers like 85 are just as silly and so it doesn’t matter if we end up with levels past 100.