I know that on international flights, UA uses a cheaper Champagne in F (and I presume C) vs. the one used in the air b/c they have to pay tax on whatever they open on the ground (don't get me started that they're too cheap to pay tax on the "good" stuff - SQ and LH seem to do it ).

However, the other day in ps F, they served me some nasty bubbly on the ground. When I commented that this wasn't the ususal ps Champagne, the FA said that this was for pre-departure, and they would give me the Chandon after takeoff. Whatever, but this struck me as odd, since there is no tax issue. By the way, that Chandon is pretty decent stuff.

(Note term "Champagne" used here loosely )