See this all the time.
The estimates are just that, estimates.
When a number of lines go live well below the estimates they will update accordingly.
Plusnet will have given you an estimate at the time of sign up. It's this that is used for your line.
They may not be able to get OpenReach to actually do anything to try get your sync higher, but should let you out of your contract if the line drops below the handback threshold.
Not much help in that every ISP using the OpenReach network/phone line will sync at the exact same speed so moving ISP won't help.
If ADSL was better it gives the option of switching back to that without penalty.