I wouldn't be at all surprised, although I would have thought they'd trial this with people who've put their hand up rather than stealth trials with users who (mostly) don't realise. EuroDOCSIS 3.0 can in theory deliver up to 1200 Mbps for the 24/8 channels of the Hub 3, and although that ultimate speed probably cannot be reliably offered across the customer base because it requires optimal line quality, there is still ample headroom to keep the Hub 3 delivering faster speeds.
I'd guess they've been trialling this to see what is possible in different locations, with a view to whether they can offer offer these higher speeds, and to judge by the figures in your list, they've been trialling you at 450, 500, 550, 600 and 650. Because a MB (megabyte) is 8 mb (megabit), the speeds in Mbps are always look higher than when measured in MBps. Looks from the cluster of speeds from 667 to 706 Mbps that may be the upper practical limit for the existing network and Hub 3. Of course, anybody hoping for a Hub 4 may be disheartened by what this implies - that VM can double existing maximum speeds without having to replace the ghastly Hub 3.
However, all of that is a complete guess on my part.