It can't read correctly because it's using a default method which doesn't exist.
Mach3 is meant to take tachometer pulses off a sensor and calculate the measured RPM number from that. That's different from the commanded target RPM of course, and part of the control loop that increases PWM pulse duty when measured RPM is below the commanded RPM.
My Hitachi X200 VFD is being controlled via Modbus. I don't have a need for Mach3's software control loop, the VFD already has a hardware control loop I've written the command to. I don't have a physical tach sensor and will not be installing one. I have a Modbus data reg with the actual RPM value already right there.
So the RPM display won't ever work as-is, the tach input doesn't exist in my system. Mach3 needs to be configured with a Brain to forward a register from the Modbus onto that RPM DRO. That's the only way it's gonna display, and it's actually much simpler... but it looks like this is just weird uncoded territory inside Mach3. There's a number for that DRO to be a node's terminator but that functionality seems unimplemented in Mach3, looks like it'll remain hooked up to the tachometer pulse integrator.