It's a recent development. Passage of Arms was altered to be the same. An attempt to make the tooltip more clear, as they used to read how you want them to read. Don't know why they changed it. Maybe to make it sound more impressive by mentioning a larger number.

Also Kaynide, as far as damage calcs go, it's the latter way you describe for everything as far as I am aware. Starting with the highest mit, they have diminishing returns on down the line. Such that you can't actually achieve 100% mitigation, making HP shields required to reduce damage to zero.

Like I remember back in Stormblood on DRK vs. Exdeath Savage, I could nullify Thunder III x2 without an immunity by stacking everything and kitchen sink with Blackest Night used as the first thunder hit. After everything, DA + Dark Mind, Shadow Wall, Rampart, Reprisal, and Addle, I only had something like +87.2% mitigation, rather than 100%.