Every radar has sidelobes.
The effect of a sidelobe is to distort the echo spatial pattern and to create an overestimate of radar reflectivity. 20 dB down is sampling a 60 dBZ echo, both will return the same power to the radar receiver. This factor of two in the returned power corresponds to a 3 dB gain in radar reflectivity. Later in this lab you will gain some experience with the effect that a 3 dB difference can have on rainrates. If a radar is poorly calibrated you can just about forget about using it to estimate rain rate.