I'm pretty sure it's just an IT problem United isn't terribly interested in fixing.
I think in every place you're offered an upgrade, there is different code in place to figure out what the price should be, and with so many ways to get into F, there's no consistency in the pricing from just buying F outright to upgrading a purchased coach fare after purchase to finding the ticket in your profile and upgrading with cash there to upgrading at check-in on website or app to upgrading at check-in at a kiosk to upgrading at the gate.... not to mention whether you're an elite upgrading to a Y/B/M fare or a non-elite upgrading to a -UP fare or just a discount F fare or....
...you get the idea.