Hi
On TTB I always get exactly 18.71 upload (sync'd at 20) and usually a bit more variable on other speed tests but always 18+. Down speeds are usually ~74 but often at peak times due to what appears to be the usual BT congestion can drop and judder around 30 or 40Mbps, however even in these cases upload speed is still 18+, I presume due to the asynchronous nature of xDSL, i.e. more is being downloaded than uploaded.
Your speeds could vary for several reasons perhaps:
1) Something on your computer or network is using data and uploading at the time of the speed test
2) Your IP stack on the computer, maybe try
resetting to defaults if Windows especially if you have tried tweaking the settings in the past. Windows 7 and above rarely need tweaking anymore, and often when you do it favours one speed test but not another.
3) Router issue, perhaps a QoS setting or CPU too busy to handle the traffic at full speed
4) Line issue, errors on the line causing packet loss and requiring a resend
5) Virus/firewall software on your computer that may be inspecting packets
then, perhaps the more likely
6) Throttling by the ISP or congestion, this will be the case if a gateway change resolves it. Are the slow downs during peak times?
Years ago with PlusNet they constantly had this issue with ADSL where the gateways would become unbalanced with more people on one than the others, and often they would drop thousands of connections to force them to re-balance. I'm sure this was only a problem because they ran the network with zero spare capacity. Not sure this now applies, but changing gateways and seeing a performance improvement takes me back to such a time.
Regards
Phil