Those choices ("26" (650A) or ATB26x2 (650B)) must come out of the manual for the bike computer. Who knows what the person writing the manual was thinking!

Here is the Bontrager tire

http://bontrager.com/model/08058

but it doesn't give such precise information. Probably it is a 559-50 tire. Here are the circumferences that Schwalbe gives:

http://www.schwalbetires.com/tech_in...#circumference

but those are just approximate. I ride on Schwalbe Marathon Supreme 26x2 = 559-50 tires and I measured more like 2010 mm circumference. It does depend on pressure, load, etc.