I could be wrong, but I think it's that different actions take different times to load in FF IV. Firaga takes longer to cast than Fire, or Bahamut longer than Chocobo, for example. From FF V onwards, every command that was given was carried out immediatly, without load times for the spells. FF XII brought this back, I believe. Or X-2 did, but I never played that, so I don't know.