This has been a problem for a while, due undoubtedly to the quality IT systems at UA. Typically, the web site is wrong, and if you get an agent to check the manifest, they'll see the correct seat assigned.

Unfortunately, there's also the case where UA's IT system or a GA has deliberately changed your seat after you already got your BP, and you'll get the dreaded long beep at the gate. Last time that happened to me, the GA said, "Oh, your seat's IFE is broken, so I moved you to another seat." Great, thanks... from an upper deck 747 seat to a middle on the main deck? Uh, no, I'll provide my own IFE anyway, thank you. Another was due to "having to move a baby out of an exit row." (Uh, then why did you move me out of an exit row seat?! Wasn't to accommodate the baby!)