Understanding Kv and max RPM limit

I am trying to understand the max rpm limitation. I previously thought that the theoretical max speed would be calculated by Kv * battery voltage. But in any case the real max speed at no load is a half of expected.

I use a vesc Hw: 4.1, Fw 3.58 at 8,4V with a Hacker A40-12S bldc (8Pols, 1350Kv). If i understood right the theoretical max. rpm is 8,4V*1350Kv=11340 RPM. I would expect the real no load rpm with 90-95% of the theoretical max. speed but i get 50%. 50% i get also with other motors and esc's ... so I guess I didn't understand something.

Can anyone explain what the real no load speed limits to a half? Are there other motor parameters that limits the max speed?