When you connect to the exchange your router will always sync at the best speed it can allowing for something called the Target SNR.
The default Target SNR is 6dB, therefore when you boot up your router, the DSLAM at the exchange and your router negotiate the best speed whilst still allowing a 6dB SNR Margin.
Now SNR varies throughout the day - generally speaking its probably at its worst in the evenings when more people are at home doing things, and more external electrical items are switched on. Therefore if you sync (connect) your router up at this time, then the negotiated speed will be lower than that when your SNR Margin is higher. (normally in the wee small hours/mornings).
The fact that your SNR Margin is now 11-12 shows that your line does vary. If you connect your router up at a time when your Margin is high - and then you will connect at better speeds.
However the warning of this is that sometimes you may sync at a good Margin, but this margin isnt sufficient to keep your line stable. A lot of routers will start to have problems when the Margin drops below 6dB... and this is where the Netgear comes into its own, because it has a good record of being able to keep hold of the connection right down to 0dB and sometimes into negative figures.
Unless you have a bad line (I notice that you have 47dB atten so your line is longish), generally 12dB would give you more than enough margin on a netgear to cope with higher speeds.
You could try a re-sync now and see what speed it connects at. If your line is a bit unstable it may well drop out later tomorrow evening and resync again, the trick is getting to know when your own lines margin is highest and lowest and working the time you connect up around that.
Theres more info about Target SNR and SNR Margin here:-
http://www.kitz.co.uk/adsl/linestats.htm
