I'm looking at booking economy ALB to MIA roundtrip in September 2013. When I search on the United site for one traveler, almost all dates are listed as 12.5K miles each way.

Increase the travelers to two adults and some dates change to be 20K.

Add a third (13yo) and many of the dates change to 30K or higher.

Huh? Can I book these one at a time? Any way to game this?

Yeah, I'm a noob. TIA.