"Max" rate is usually the rate would expect to get with the current signal quality at your target snr.
Very roughly, the snr is the amount of 'extra' signal needed to cover noise problems.
The target snr is almost always 6db. That is the snr it tries to get when it makes a connection.
If you have a higher snr, that means more stability but slower speeds.
The stats when you took them show 6.4 snr downstream; the line must have been a little better then than when the previous connection was made.
If the line is periodically noisy, it is most likely to lose connection when the noise is high.
When it tries to reconnect it is doing it with a bad signal, so the connection will be on the slow side.
You can force a reconnection when the signal is good to get best speed and you should get a lot closer to your 49Mbps;
that runs a significant risk of broken connection during the next noisy period (and reconnection back nearer 40Mbps).
When I am talking connection here, that is the connection between the modem and the cabinet; not the higher level one from router into the internet.
Whatever you do, don't force a reconnection for the modem by unplugging and replugging the phone connection.
If possible, disconnect/reconnect via the modem interface, if not by power down, wait a bit, and power up.
Any disconnect/reconnect may be mistinterpreted by DLM as noise, which will keep you on interleaved for longer.
I'm not sure what levels of errors trigger interleaving so can't help you on that one.
Once DLM does introduce interleaving, it can take three weeks or so before it removes it even if the line is near perfect.
Recently moved from BT Infinity 2 to PlusNet. Very happy so far.