There have been situations of some dude farming up to like 2100 rating or so just from having like a 100% win rate against his two friends that are horrible. Good old TuxedoSam.
1500 is the expected median, if you have some artificial environment where it consists of like 4 dudes, even if they are like 200, 300, 600, and 1000 in the general FAF ecosystem if they purely play each other in custom games they could be grossly different.