User blog comment:B214/Shin DxD 1 discussion/@comment-95.210.189.106-20180809123512/@comment-28884330-20180809220118

I didn't "make" Issei's level. Issei himself said in Volume 24 that Vali's DxD L was superior to him, it's not something I made up.

Also, even if Issei has more evolutions, Vali has a better base as of now, so Issei needs more evolutions than Vali just to put them on par with each other.