Making Sense of Volo’s Evoker CR

The Evoker in Volo’s is a CR 9 and I just can’t figure out how to calculate it up to that level.

AC: 15 (with Mage Armor) & HP: 66 with 2 Saves = Defensive CR 1.5

If we added in that Stoneskin is active prior to the start for resistances = Defensive CR 3

Spell Attack: +7 & Save DC: 15 with 75 DPR average = Offensive CR 11

(( Cone of Cold – 1 6th Level, 2 5th Level))

So the best I can get is CR 6 w/o Stoneskin and a CR 7 with Stoneskin.

Not sure how to boost it to a CR 9 in my studies. Was planning on making a slightly stronger Evoker but trying to get the baseline down.