I think the problem is when the timer for the site starts later than the upload does. So you get this strange situation where if it thinks it starts by the time half the data has been sent then your speed looks twice as high as it should. I have seen this before, but only on connections where people had software issues!
---- I do not work for VM, but I would. It is just a Job.
I would also make websites for them, because the job never seems to require the website to work.