Hmmmm....that sounds like a very nice GA. Definitely don't expect that treatment on a regular basis. You'll just feel let down when the next GA balks at the question. Most freebie *A upgrades are given according to status and only occur in oversell situations. Given that you said C was only 1/2 full, I'm assuming the GA was just feeling friendly. Without using a GPU or miles, there's no basis for the upgrade to be processed.

Domestically, you'll find that United's Premier Silver status yields an extremely low upgrade/CPU rate. You can read up more on upgrade policies on United's website >> Mileage Plus >> Upgrades Overview.