Has anyone else been able to reproduce this since OP's original posting?
I am used to seeing prices bounce around, and had attributed this to, well, ... prices bouncing around. It had not occurred to me that it might be tied to whether I was logged in or not, and I really appreciate the OP's effort in nailing down what was going on in this instance.
So far, in some fairly unsystematic efforts for some familiar routes, I have not seen this happen, thus my question. I'm curious as to how prevalent this glitch (or policy) is on US's part ....