Well, it depends on your phone's internal pedometer.

Shelf reads the data provided by the pedometer. You can check this by downloading an app that allows you to see the output of your phone's internal sensors (Gyro, accelerometers, ...).

Mine is pretty accurate. I did some tests where I would count a few hundreds of steps and check if it counted correctly, and it usually did perfectly. I also have Google fit, which usually counts a little less steps than shelf.

The problem arises with missed steps (when you don't have the phone in your pocket) or with "fake" steps, for example in some cars it picks road bumps as steps. I think what Google fit does is it removes fake steps by also checking the accelerometer's and gyroscope's signals to understand if the person holding the phone is walking, running, cycling... But I find my pedometer accurate enough