The BT IP Profile is held at two places if not more, and these can get out of step. One is the BRAS, the other is the MSAN. Presumably one is a copy of the other and the copy routine failed the last time it changed.
This came to light a few years ago when the system Plusnet run of also holding a copy themselves (a system where their copying is known to fail frequently) looked correct as it agreed with the value reported by the BT speed test, but the throughput was clearly more restricted.
They ran some other test that showed "an imbalance in the BTW system". You seem to have the opposite effect, as you can exceed the reported IP Profile. It might be worthwhile asking them about this test they can run. I'm sorry, off-hand I don't know its name
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