There was a big push for the past several months to get people to buy advance tickets. It's difficult for some people to keep reservations made so far in advance.

Just an anecdotal story, but my aunt and uncle bought advance tickets for July 5th (the day after it opened), but couldn't go because my uncle busted his knee playing basketball on July 4th. And they weren't able to exchange their tickets for another time, and weren't able to find someone who wanted to go to give them to.

So maybe something similar happened to explain the empty seats you saw.

The upside is that people who bought advance tickets, but didn't use them, are most likely going to buy new tickets because they are dedicated to seeing that movie.