Why do Chenvaala and Kael'thas count spells differently?
Chenvaala needs to see you cast three spells.
Kael needs to see you cast a third spell.
Hearthstone's wording isn't always perfect but generally speaking in card games different words = different effect.Fuente
As Marrow said. Kaelthas's Verdant Spheres were probably the most contentious single-effecr spell I was ever involved in, because flavor-wise they're dope but they didn't actually follow the rules for pips. Either we had three spheres - rad, but the third would always be empty, or it's glow obnoxiously, or some other weird thing - or you only have two spheres that behave consistently with other pips but then it's just... not the hitting fantasy anymore, and still sort of weird because the card says three, the pips say two, yadda yadda. There was just nothing with pips that both felt right and followed the rules.
It's part of why we made sure to get them in on his BGs hero power where it did work with the mechanic, we really wanted the spheres to make it somewhere lol.