I did see your post this morning. I just didnt have time to type a reply before I had to go out, because I knew it wasnt a 2 minute reply. :/
>>> I wonder what is cause of these differences of snr margin measure.
SNRm fluctuations are normal, I've passed the links a few times before for SNR, SNR Margin & Target SNR. If you have a read
here it may help you to understand what is going on.
>>> I think that lower power output cause lower snr margin.
As mentioned above, power output will have an effect on your SNRm & Increasing the power output will increase SNR figures, so yes you are correct in that lower output power will cause lower SNR. However bear in mind that SNRm fluctuates normally without changes in power. The Power cut back is there to prevent SNRm getting to high and your signal getting too strong and drowning out neighbouring signals.
There are certain power cut back profiles, however no-one knows what the power cut back parameters actually are. They are an adsl standard that has to be there, but are industry standard masks and profiles which may vary from country to country and line length.
>>> Can't devices correctly measue snr margin?
Devices do a pretty good job at measuring the
Signal to Noise RAtio, so yes the SNR Margin should be fairly accurate.
>>> Is it possible to increase snr margin by new synchronization?
Yes and No
On the face of it, it may appear to change as your router reports it.. but the reality is, that because its linked to your REAL SNR which definitely isnt affected by new syncs, all youve done is adjust other parameters so that the SNRm gets as near as possible to the Target SNR.
Resync is one of the times when you may see a change in power output though, as this is one of the times when a check is made to ensure your signal isnt too strong.