It is possible that the system you are on is a bit like it is in the UK.
In broad terms, here the ADSL2+ comes from equipment in the telephone exchange along with the telephone service, with a cabinet (Primary Connection Point or PCP) nearer to your premises where a cable carrying many copper pairs is connected to the individual lines to premises. The attenuation measures the signal loss. The SNR margin is usually set to 6dB by that exchange equipment.
With VDSL a separate fibre cable is run to another cabinet near to your PCP. The two cabinets are linked and the phone signal is merged with the fibre signal, giving phone plus VDSL (VDSL2 in the UK) on copper to your premises. (Recently there are some cabinets containing the combined equipment so you may have those).
From your post it sounds as if the ADSL2+ continued to be sent to you as before but passing straight through the VDSL2 cabinet. When you got your VDSL modem that was able to pick up the new signal generated much closer to you, at the cabinets instead of the exchange.
With respect to the SNR margin it is possible that your VDSL could supply much more than 20Mbps at your premises but is capped at the cabinets. If you are fairly close to the cabinets then that capping would result in a higher SNR margin. If you were quite a distance from them so the signal attenuates/weakens more, the SNR margin would also fall.
Have a read of the UK-targeted SNR/SNRM page
on my website for the general principle, and Cause 3 here
. Again, although that is ADSL the same principle applies.
The FTTx page may also help.
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Edited by RobertoS (Fri 24-Mar-17 09:50:32)