No way to tell what strategy is best. From my days as a CSR working US-MEX flights, I would say UA has a good strategy here. I recall flights booked real light a week out filling up quickly, and the immigration pattern of USA-Mexico during Christmas is when lots of seasonal/migrant workers return home bringing gifts and money. (I believe money from relatives in the US is 1 of the top 3 sources of income for Mexico with tourism and oil being the other 2.)

So when you combine ultra-high season with a short booking window, selling all seats for alot sure beats selling many for little with only a few for alot. Plus "alot" in the US-Mexico market isn't really all that much.