To me, it sounds like it might be at least in part an artifact of the level cap. When you have fewer levels available, each one you give up costs you more than if the cap is higher. Dropping four levels into a second class (so you don't lose an ASI) costs you 1 maximum spell level in tabletop as you can now get up to 8th level spells instead of 9th, but 2 maximum spell levels in BG3 as you top out at 4th instead of 6th.
