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The P25 audio is digital, so it degrades worse than analog audio when signal quality is bad. You more quickly get to a point where it's not understandable at all, particularly if it's the signal coming from a handheld radio that's some distance away from the receiver.

The analog repeater is likely getting a high quality feed from the digital system, and then that's being broadcast at decent power from a tall antenna. It's starting with the best audio the system has to offer, has advantages in how it's broadcast, and degrades more gracefully.

I capture a lot of the P25 traffic in my area and there are times I can understand the dispatcher side of a conversation fine but the other end is too far away / too weak a signal and is unintelligible. The dispatcher's signal is coming from a fixed tower transmitting at higher power than a portable radio can manage.