Check is to look in your hub's status page, seek out the Configuration page, and scroll down and find the Primary Downstream Service Flow section, and the entry Max Traffic Rate, which is in bits per second (not megabits, so knock off the last six digits to get Mbps). That's probably set to the circa 549 Mbps that VM use for the 500 Mbps tier (all VM speeds are set at about 10% over the headline value, never say they aren't generous on a few things!).
There is one possible caveat, because in the past VM have done unannounced customer tests where they just zapped up individual or local user speeds for a week or two, for the purposes of network testing, and often done these at different levels before quietly resetting the MTR to normal. The speeds you mention would only be feasible with a Hub 4. So if you've got a Hub 3, Mike will be right that it's an error by the speed test site, if you've got a Hub 4 its still most likely an error but it certainly could be some network testing, although if they've finished the field test then MTR will have been reset to normal, and there will be no evidence. Some years back there were unannounced field tests running a few lucky users Hub 3's up to 700 Mbps, and there have certainly been a couple of announced trials before with the Hub 4 running up to 2.2-2.5 Gbps in selected locations. VM will need to run further field tests to see what their next step is above 1 Gbps, so field testing will be going on, but the company probably won't ever admit to such field trials for reasons of confidentiality.
From a marketing point of view, 1 Gbps speed is now last years news, and available in selected locations from a range of other providers, so VM will be looking for the next big thing, although they'll want to have completed or be close to full network roll out of the DOCSIS 3.1 standard to support 1 Gbps and higher speeds, which was originally slated for completion this year. I'd also expect a progressive sales and marketing push to get customers onto 1 Gbps speeds - investors will be asking why VM have spent billions rolling out 1 Gbps if the take up is poor, and with currently very few domestic uses that need such speeds, it will have to be pushed. A pretty similar question is already being asked about Openreach's FTTP programme, where the industry working group attribute lack of take up to consumer ignorance, rather than the obvious answer that most households can't full saturate even a 100 Mbps connection, so think "why pay more for speeds I'll never use?"
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