Anyone know what the Max Achievable Speed on BT Speedtester means?

[#1]

I am on ADSL2+ and can only sync around 4.2Mb (from 3Mb initially thanks to Jojo) but according to Kitz I should be nearer 6Mb. My BT Speedtester http://speedtester.bt.com/ results below show a Max Achievable Speed of 4000Kbps and 

Quote

For your connection, the acceptable range of speeds is 1200-4000 Kbps.

I wondered how it might arrive at that figure and what other members results show.
Geoff

[#2]

My opinion, for what it's worth, is that BT introduced that strip because users complained they were on an "up to 8Mbps" or "up to 20Mbps" service but only getting (say) 3Mbps, not appreciating that was because of their distance from the exchange. Suspicion that their ISP was trying to con them into accepting an inferior service.
However draw a strip showing 3Mbps within 4Mbps "maximum achievable" and users are more inclined to accept reality - "the computer says so".
From what I've seen in posts, I think the speedtester has been programmed with strips covering a number of ranges, one of which extends to 4000kbps, probably selected because the IP Profile is 3500kbps. If the IP Profile was higher (or lower) a different strip could be drawn. Basically I think it's cosmetic and provides no technical information on what performance the line might or might not be capable of (eg for fault diagnostics). Router stats (including error rates) are the best end-user guide to what a line might be capable of.

[#3]

Thanks for that spraxyt. Ah well I guess there isn't a mysterious limitation on my line then.
Cheers,
Geoff

[#4]

Just for information, if you are on a 'Banded Profile' then the maximum achievable is related to the banding (or it was when I got banded).