RPM Gauge not equal to RPM value

Hi members,
On the dyno we noticed that the rpm gauge isnt equal to the current rpm value at any time.
The gauge shows 5000 rpm and the value is around 4300 - 4700 rpm.
How can this be?
This also happens on higher rpms.
On other gauges like engine load normalised this doesnt happen like this. The gauge is exactly the same value.
Thanks
Malik
On the dyno we noticed that the rpm gauge isnt equal to the current rpm value at any time.
The gauge shows 5000 rpm and the value is around 4300 - 4700 rpm.
How can this be?
This also happens on higher rpms.
On other gauges like engine load normalised this doesnt happen like this. The gauge is exactly the same value.
Thanks
Malik