Spinning Speed and Effort Does not Match Virtual Speed

I’m relatively new to Zwift. I have noticed in particular on parts of certain rides that the speed shown on the screen does not seem to match the speed with which I am turning the pedals and the effort I make. When this happens, the virtual speed is slower than it should be.

Most of the time the virtual speed seems accurate, but about 10% of the time the speed is off; more on certain courses.

Anything I might be doing wrong or any calibration I could be doing?

Read his:How Does Zwift Determine My Speed?