An Atlas location is composed of a position (flag pin) and a radius (green circle). It can happen that the actual position is not inside the radius circle.
Actually, 90% of messages will be inside the accuracy circle. It means that 10% of the messages can be outside the radius circle.
It can be explained with conditions being not utmost, e.g. high altitude, speed > 5 km/h.
Another possible cause is when a message is received by several base stations, one of them having a high RSSI (ex: -70 dBm) whereas other stations have a low RSSI. In such a case, one can expect the position and the radius to be accurate. It will be accurate, except when the high RSSI base station is not used for the location.