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If your line wasn't great then yes, your profile would drop (by itself, no need to reboot the modem). Profile rate is always a few percent lower than your line rate, sometimes a lot lower if there's been recent faults.

Speedtests on the other hand vary based on a lot of factors, including congestion and performance of intermediate devices which faster connections tend to reveal. My DIR-615 for example, did fine on 40mb but was too slow to handle 80, though with QoS off it would just about hit 90 on a good day. The Openreach supplied modem additionally is too slow to handle >60mbps in router mode.