As you already know attenuation should more or less always remain static.
As eric has already stated, it can be affected by different routers due to the way they record the frequency bins, or it can increase if you get higher speeds and therefore use the higher frequencies which are more attenuated (eg if you go to adsl2+).
A decrease in attenuation is normally different equipment on the line (router), improvement of internal wiring, a repair on the physical phone line or.. a new twisted pair.
TBH although O2 may perform a woosh equivalent, I wouldnt expect them to find anything much since it will be your router that responds to the tests and will likely give similar figures.
If it wasnt for the time at which this occurred, I would have said most likely some work has been carried out on your line which has improved the attenuation such as a new twisted pair or linecard.