You have a misunderstanding of how the FTTC system works and how the information about how your line should perform is reported, if you signed up expecting around 50Mb , ( you are on Fibre 1 ) some on this package will get the ‘full’ 55Mb speed product limit some will get less , FTTC ( fibre to the cabinet ) is rate adaptive, the longer the line from the cabinet the slower the speed , plus line defects and poorly installed or faulty internal wiring ( the first thing Openreach responsibility, the second thing the customer’s responsibility) can affect line speed …when you sign up , it’s an estimate of what your line should achieve based on a mathematical model, after you have been a customer , there is actual line performance data to base the speed range on, but no one has altered anything , your line is exactly the same as it was and will perform pretty much as it did regardless of the speed estimate,
Your product ( Fibre 1 ) hasn’t changed , your line hasn’t changed , no reason to think the speed you get will change.
There is the possibility that as more customers join your cabinet there is more ‘crosstalk’ which could affect speed but again this is simply a consequence of how the system works not a manual intervention aimed at you and isn’t a change to your product.
Its possible that the real data the ‘new’ estimate is based on is lower than the first estimate , because there is an issue that needs addressing, like a line fault , ( causing a reduction in speed rather that killing it ) the estimates are really there so should the performance fall to below that level it’s an indication that something may be wrong, your router stats will give a better indication of your lines potential and actual performance
