Issues with 2 motors in series
Could it be that the faster motor is actually different in some way?


Yes. Due to manufacturing tolerances and process variations, no two motors are identical. If one motor draws slightly more current for any reason (higher brush or bearing friction, greater load etc.) the other motor will speed up and produce more voltage, causing the 'greedier' motor to slow down. 

It can work if the motors are well matched and the loads increase greatly with rpm (eg. propellers), or they share a common load (eg. coupled to the same gearbox, individual wheels on a road vehicle).