OK, I may be wrong here but:

ADSL2+ uses additional frequencies than ADSL and those frequencies are in a higher range. The attentuation figures are calculated based on the whole range of frequencies. Higher frequencies over any medium attenuate at a higher rate than lower frequencies (which is why mobile phone companies prefer lower frequencies because they travel better through air and building structures).

So, when the higher frequencies are added in to the attenuation map it results in a higher attenuation overall.

Now that could of course be complete rubbish but I am pretty sure it is not.

In reply to a post by RobinsonUK:

I can't believe the line rental price: £14.50 per month? It's kind-of annoying that I have to pay a "rental" for the line at all. I mean I don't pay "rent" for my mobile. Without the line they can't deliver the broadband anyway!

The cost of installing and maintaining the copper pair has to be recovered somewhere - would you like it included in your ADSL charges? they would then be a lot higher each month.

You can, of course, pay BT line rental for a year up front which works out at about £10.50 or so (or it did when I did mine).

There is a difference between telephone line rental which pays for the rental and maintenance of the physical line and exchange equipment and a mobile phone where if you don't use the phone you incur minimal costs to the provider.

Whether the mechanism of charging of line rental untimately to OR is a matter for much debate. Personally I would prefer if OpenReach provided the line, that you paid for, and any services (including voice) where chargeable by the service provider. This would make "naked DSL" and "full fibre" much easier to provide and would make VOIP a viable technology.

Spot on.
Some routers may actually report the attenuation at 300KHz, rather than across the whole spectrum in use too.

The 2wire 2700/2701 family report Downstream Attenuation at 300kHz and also the Attenuation across the whole of the band that is in use which is fairly dynamic.

Similarly the noise level is normally measured across the full spectrum and that used to give the SNR/Margin. I did see one non-UK version of 2wire User Interface which reported noise figures and several specific frequencies - in both up and down bands.