Yes. You are very right.

Flowmeters are one tough piece of equipment to properly calibrate and maintain. Every thing from the Reynolds number to temperature and viscosity can introduce errors. This is more so with the non-positive displacement types ( aka Turbine types ).

However with the advent of the positive displacement types ( aka Gear flowmeters) these variations have been reduced to a very large extent. 0.1% accuracies are common with these flowmeters.

Raghu