Delta Printer - Z axis 20% undersize

Hi,
I've just replaced the motherboard in my Tripodmaker Black with an SKR V1.3 and finally have it printing. I have the delta printer height set correctly, my diagonal rod length is correct and my steps per rotation is also correct. When printing a 20mm calibration cube though, the X and Y are 19.1mm but the height is only 16.6mm. When I auto calibrate the printer, it returns the correct height; is there anything anyone can think would be wrong?

Z steps per mm seem off.
I have a delta and mine uses the same steps/mm for X, Y, & Z.

You also said that the X & Y dimensions are ~5% undersized so calibration is obviously not correct in any dimension..

I don't use auto calibration so cannot speak to the differences between manual & auto calibration, but mine is spot on in all 3 dimensions.

My controller is a Makerbase MKS gen L ver 2.1 board using reprap firmware.