So I found this guide online: https://diablo2.diablowiki.net/Guide:High_Level_Baal_Runs_to_99_v1.10,_by_jjscud_and_Reborn

It notes 20 baal runs for 90 to 91 however when I do the math of actual experienced after each run, i'm getting 31.6 runs and that's for 89 to 90.

Have the maths changed since this guide came out or am I just doing something wrong, lol.

I would suggest looking at the stickied post from bjones9 at the top of the BR forum for exp details. Nothing has changed.