It appears as though you were #1 on the UG list because those with status higher than you had apparently already boarded and, thus, their name was removed from the UG list. Had you held back... in theory, you should have gotten the UG since, as it turns out, there were two seats available in FC. But you boarded instead (on bad advice) and were thus removed from the list leaving the two FC seats to go to others.

I don't see the harm in bringing this to Delta's attention. Upgrades are one of the biggest perks to elite status and one of the main reasons most of us strive to maintain status with Delta. Losing out on an UG that you feel like you should have gotten is frustrating, and could potentially drive your business to another airline with a more transparent (or at least consistent) UG policy at the gate.