Providers don't set the SNRM target. OpenReach control it via their DLM (Dynamic Line Management).
The upstream is always say to a 6dB target.
The downstream is 6dB, but on Huawei cabinets it can be lowered to 5, 4 or 3dB.
The SNRM target is never above 6dB on both downstream and upstream.
What your stats show is the current SNRM at that moment in time, not the target set.
Your current downstream SNRM is on target at 6dB.
Your upstream is above the target as the line isn't syncing as high as it could.
It is capped at 20Mb by OpenReach as that's the max they offer on VDSL2/FTTC. If it wasn't capped you could get around 29Mb.
The result of your upstream syncing under what it's capable of is the current SNRM is above the 6dB target. So you have "spare" SNRM.
I forgot to mention, I was talking about the downstream SNR which goes to 9 and the speed drops but if at 6 the speed goes back to normal. Hopefully it will stay as it is for now but the information I was looking at was here
https://www.increasebroadbandspeed.co.uk/SNR-tweak which says 9 is TalkTalks default.