BT Speedtest oddity

[#1]

Ran a BT Speedtest this morning. It reports that my IP Profile is 4.5Mbps, while at the same time indicating that the Max Achievable Speed is 4Mbps.
This is not making sense to me, especially as various speed testers confirm that throughput is around 4.25 Mbps.  

[#2]

I don't think that strip has any technically based significance. All that can be said is the "Max Achievable Speed" will be larger than the actual measurement you've just been presented with.
You will see instances where a measurement of (say) 0.2Mbps is accompanied by Max Attainable Speed 0.25Mbps even when the sync speed (no longer shown on 21CN) is 4Mbps or more.
I think the strip was added to try to stop user complaints that they were on an 8Mbps service and not getting anything near that. Marketing guidelines have changed significantly since those days to try to set better expectations.

[#3]

Fair enough. Well, it is BT.
Thanks spraxyt.