I doubt if any members (in the U.K.) will be able to give you a explanation.
Perhaps you could ask Draytek directly about the Vigor 2762? Looking at the specifications of the version that is available for the U.K. and Irish markets it just mentions "VDSL2 Profile: 8a, 8b, 8c, 8d, 12a, 12b, 17a, 30a". I do not see any obvious mention of a 100 Mbps limit for Profile 30a.
There is one difference between Profile 30a and all of the other VDSL2 profiles -- the sub-carriers (tones) are spaced every 8.625 kHz rather then every 4.3125 kHz, as with Profiles 8a to 17a.