Priority is an odd duck... USPS is also...
How it was explained to me:
All mail from our facility goes to Charlotte hub. Items within XXX miles regardless of class are trucked PO hub to PO hub via USPS.
Items over XXX miles are contracted out, usually to ups or fedex to haul by ground. Once on a truck, they bounce between hubs and eventually make it to your PO (sometimes routed efficiently, sometimes not)..
(xxx= I can't remember the exact miles)
The difference between priortiy and first class is that priority, once deemed over the magic xxx miles away, is flown instead of driven to the local hub/PO. The best value on priority is from say Colorado and west where the delivery time is 2-3 biz days compared with 3-7 biz days for first class.
USPS also has time limits on delivering priority mail where they typically don't on first class mail so priority mail is always scanned when they deliver it.. But if they scan it late, internally they get penalized so sometimes you may see a scan that the package is delivered and you don't get it until the next day and it's typically because the driver forgot, missed it or for some other reason couldn't deliver it that day but has to scan it as delivered to not miss thier delivery time number..
Take this with a grain o'
salt because it's just a summary of the parts of what I've been told by ups, usps, mail carriers and others involved..
hoog