I've noticed the same thing, but check if there are codeshares on UA - I've been getting flights for one of my routes far, far cheaper than US sells them:
As an example:
5/22 PHL-CLT-SAV on US1593/US2258 - $435
On UA stock:
5/22 PHL-CLT-SAV on UA2633/UA2101 - $145
Same metal.