It’s not as much in the tuning, but remember that’s something that happens in general with most customs and the idea of a “perfect seal.”
I will say - 64 audio is notorious for some reason, having different tunings on their CIEMs versus universals. I’m not sure if this the case for the U12t vs A12t.
Hope someone like @Precogvision will be able to chime in with more info.