The max speed of the axis might be acceleration limited. For example:
SPEED = 800
ACCEL = 1200
DECEL = 1200
Say the vector length = 600 - 500 = 100. ie the MOVE length is 100.
From the standard equation of motion:
v^2 = u^2 + 2as
u is 0
v = SQR(2 * ACCEL * length)
v = SQR(2 * 1200 * 100)
v = 490
So it will not go to the 800 set speed.