Speed variation after upgrade

My local exchange (Hornsea) was upgraded a couple of weeks ago to 21cn. For the first few days download speeds jumped from about 6 to nearly 15 at times. They have now dropped back to the previous speeds but the bt speedtester says my IP profile is 16 and shows a download speed of about 14. I usually use the speedtester on this site and at speedtest.net.
Why the sudden drop in speeds? I use wireless but have not altered any settings.

Tried my wife's computer on ethernet and speeds are way uo to 14! Must be something to do with the wireless connection but why the difference in speed tests on here and on the bt test site?

In reply to a post by Zurdo:

Tried my wife's computer on ethernet and speeds are way uo to 14! Must be something to do with the wireless connection but why the difference in speed tests on here and on the bt test site?

Two lessons;
1. Wireless is hopeless.
2. Speedtest sites are notoriously inaccurate (and easily overloaded).

The BT test is able to bypass general internet (and ISP) congestion, and test just your line back to the OR equipment. Every other speedtest site also reflects "issues" with the internet, ISP back-bones, etc.
Find a speedtest site and stick to it (and get comparisons between different tests on the same site).
The Flash based testers normally give high results (sometimes higher than the line speed).
The Java based ones seem to give more consistent results, but there are so few of them (I've only found two which are in the UK and, thus, able to give 74Mbps results for an 80Mbps line).

There is no better way [to test download speed] than actually downloading a file.
A single download thread (what you get when downloading through your browser) will normally give a lower speed than a multi thread download (using download manager software). The difference is the key to resolving your problem (if it is, indeed, your problem and not just ISP throttling or congestion).

Speeds are back up again. Hope it lasts!