Yes, really
Virgins percentage difference is much larger than the others when you look at the maximum speed and average speed numbers quoted and then compare that to the evening peak time hours. You are just comparing the headline speed of both services with the evening speed.
If you go by the highest maximum average of the services and then the same at peak times, Virgin slows down almost 12% compared to FTTC of 6%. Similar results if you take those values from the 24 hour average too.