It's a common practice with airlines, and other travel providers. As fti it's how the fares are allocated.

When you have multiple fares, if they can not all get the same fare, they will all price at the higher fare. It's totally possible you could do two bookings of two people each and get them for $840 per person, or it's possible you will get two for $840 and two at the higher rate. It's always best to look at groups of tickets also as individual tickets to see if there's a price difference (and if there is it enough to make it worth it to have separate tickets)