The router sync rate has no bearing upon the ability (or otherwise) to reach certain arbitrary sites upon the internet - and this is as far as these speed test sites are, since they're just measuring your bandwidth between their server (which could be anywhere on t'internet) and your PC.
Router synching on BT Wholesale ADSLMax (e.g. up to 8Mb) and resold products thereof will not sync at anything better than 7mbit/s due to their traffic shaping (and yes, that's a whole can of worms for the ASA to deliberate over).
Your varying bandwidth reports are more likely to do with a combination of the following factors:
1) Your local exchange equipment
2) Your local exchange's bandwidth to the colossus network
3) Whether your exchange has been converted to 21CN or not
4) Your ISP's trunk to colossus
5) Your ISP's trunks to other internet peers
6) The time of the day
7) How many other people are downloading illegal films at the same time as you
8) Whether your PC is running a lot of network-related items in the background
9) Your network contention ratio
(and several other spurious reasons).
HTH.
Mike.