Back then (in BC), they changed that. ilvl up to around 90 was linear, but during BC, they changed the stat/ilvl formula. Starting then, you had the "modern" formula of 15 ilvl = +10% stat (30ilvl = +21%, 45ilvl = +33%, 60ilvl = +46%, and so on), which meant they just had to plug "next tier is...