The IP profile is something artificially implemented by BT wholesale to rate limit traffic to each line as it is something BT have long believed necessary on their services. It is separate to the natural overhead of the connection protocol which is PPPoE for BTw based FTTC services.
That was true in the days of ADSLmax, with the stepped nature of the IP profile, exacerbated by the slow restoration time - both aspects were indeed artificial. And delays incurred within Plusnet will have further exacerbated problems.
But the artificialness went away with the immediate updates in both 21CN and FTTC, with the values reflecting real overheads.
Unfortunately, delays still occur within Plusnet, making issues appear there still. Added to the fact that they (appear to) leave room for their QoS prioritisation work.
But, overall, the modern IP Profile is no longer the artificial tool it once was.
After all, your line can indeed only cope with a certain rate. If more data has to be sent, it can only ever be handled in one of two ways - queued (causing bufferbloat and jitter) or dropped (the right answer, for TCP).