I am also having a similar problem to this. I have recently joined Virgin Media and have the 1GB broadband service (1000x50). When I run the SamKnows speed test (on WiFi or Ethernet) I receive around 1154Mbps Download to the Virgin Media Hub 4; but only around 500Mbps to the device. An example device is my Apple iPhone 12 Pro; but I have also tested with a year old Dell i5 laptop too.
The strange thing is my Dad has the slower speed connection at his house (600x40), and when I did the SamKnows speed test on the same device there (on my iPhone 12 pro) I was getting 649Mbps Download to the Hub (he has a Hub 5, but previously had a Hub 3) and 645Mbps to the device. Therefore I'm getting a higher download speed but on the lower tariff.
I have tried to keep my testing the same (e.g. similar distances to the router), both were done at night last night.
I have tried to check/work through some of the other suggestions on some of the posts, but I couldn't see anything obviously wrong in the config. Does anybody have any advice to increase/resolve this?