Something to think about in all this is how the 0-5V and 0-10V conversion works. Generally, it is what they call a Low Pass filter which means that the top voltage (5V or 10V) is based on the highest voltage of the PWM signal. A lot of power supplies are within spec at plus or minus 5% so that PWM signal may only be able to provide a 0-4.75V signal (95% of 5V). So, if your "5V" PWM signal comes from a circuit with 4.75V, it will never get to 5V, even at full speed (ie 100% PWM duty cycle). To state the obvious, it would only get to 4.75V, at best. Also, digital logic often eats a little of the voltage so you might also be seeing that. You can measure the net result by outputting a full speed signal and measuring the voltage of the PWM output with a halfway decent multimeter.

A lot VFDs which accept PWM input directly use the Low Pass Filter approach and have the same issue.

The 0-10V conversion modules you can get are often adjustable and allow you to compensate for the source PWM signal being low. Measure full speed output of it with your multimeter and turn the potentiometer until it reads 10V.

Or, you can just accept that 16K RPM is really just 15.2K (or what ever your output voltage is off by).