Your connection speed determines your
BT IP profile. The profile does not determine the connection speed.
With most ISPs, but not Plusnet, your BT IP profile directly determines your maximum actual download speed.
A lower connection speed at any time triggers a rapid fall in profile to the appropriate level.
A higher connection speed at any time causes a rise in profile, but takes from a few hours to 5 days.
When your BT IP Profile changes, Plusnet are supposed to get an update of a copy of it that is held in the Plusnet system. Sometimes that update doesn't happen, so your Plusnet IP Profile can be out of date, either higher or lower than your current BT IP Profile.
If the update fails, the Plusnet copy will only be corrected by either a successful update next time your BT one changes, or by Plusnet support manually adjusting the copy.
When your BT and Plusnet copy IP Profiles are out of step, your maximum actual download speed is determined by the lower of the two.
There is a fuller description of the BT IP Profile system on
this page of my website.