Not that it really matters but just done a BT ADSL checker test. It's about the same for fibre as PN.
PlusNet probably apply a small amount of rounding to the figures obtained from BT.
In the downstream direction, the checker speed is slightly less than you actually manage, but by less than 2Mbps - or around 4%. That's a pretty good prediction.
In the upstream direction, the checker speed was pessimistic - you manage 1.9Mbps more than predicted.