There are only 5 Epidemic cards in the game, so it makes sense to have 6 positions on the infection track. However, it has 7. Did we miss something?
We've completed the game already but there were no new rules in regards to this.
You didn't miss anything. In the regular version of Pandemic you can play with 6 epidemics. Presumably they left the 7th position to allow people to guess that a 6th epidemic may show up.