Whilst specific to Virgin, this post on tbb is a good summary of why multiple thread speedtests (BT, speedtest.net et al) can show good throughput whilst the TBB speedtest, which shows a single thread result and a multiple thread result, can show bad and good results at the same time. And it is not necessarily due to purported whitelisting of said sites.
http://forums.thinkbroadband.com/virgin ... t=all&vc=1
Looking at some of the complaints about speed, I would hazard a guess that, for example, Sky on demand downloads are single threaded - hence the vast difference between off peak and peak time performance reported.
I post this for the sole reason that the post above contains info that might remove some confusion as to how some are having issues and some would appear content ....