Let me explain what happened.

I was going to meld a Savage Aim Materia IV onto a darksteel haub, but the game said that "the selected item's level is too low to maximize the materia's potential" (Critical Hit Rate + 0 in red writing).

However, on the bazaar there are several darksteel haubs with tier IV materia melded to them.

Again, I must be missing something obvious, but I'd appreciate it if someone could point out what that obvious thing is.