BUT!
I have my steppers set at a speed of 140ipm.
The DRO reads out 152ipm when I rapid move.
Turns out the frequency is actually 27,300 pulses (Not 25,000)
There's your answer right there in the numbers.
27300 / 25000 = 1.092
1.092 * 140 = 152.88
Unless your PC shows a rock steady exact 25000 pulse frequency (which I'd be surprised if anyone's does) then this is what happens.
Perhaps more curiously, when I FIRST start Mach, the actual pulse frequency is a rock steady 25,324. However if I close and restart, this drops to a rock steady 24,397. If I close and restart again it's rock steady at 23,674. Closing/re-starting from then on has no further effect and it remains at 23,674. If I re-boot then the whole (3) cycle will repeat exactly. Just one of those Mach3 things I suppose.