The longer the range for the radar to 'see', the slower the rotation rate. Most en-route radars rotate at around 7.5 RPM (individual sites range from 7 to 7.6). Shared en-route and airfield facilities will be in the order of 10 RPM (sites range from 9.72 to 12). Radars primarily used for airfield ATC will be in the order of 15 RPM (sites range from 15 to 15.7).
The exact rotation rate is known and controlled by the ATC control and monitoring systems and the RPM will not vary considerably as you state. More likely that the inaccuracy lies with the 3rd party software being used to work it out for the non ATC application you illustrate, which is not required to have the same accuracy as a regulated ATC system.