Several reasons can explain the discrepancy of the speed illustrated on the map and the speed measured by quality measuring tools like Checkmynet.lu : (1) You are inside a building, (2) your mobile phone does not support certain configurations (frequency bands), (3) the cell usage load at the base station is too high, (4) the propagation of the signal is negatively affected (weather conditions, physical area, drop) or other factors that justify that the speed indicated on the coverage maps is not always reached.