Bit of a weird one, and I'll try and explain.
I don't have a short line from the exchange, or at least to my knowledge. Historically, it has only been possible to get no more than ~8 Mbps sync speed on ADSL2+ in this area. The other line, when it was ADSL2+, used to attain a maximum of around 8.3 Mbps at 6 dB SNRM fastpath (3 dB would be ill advised, unless interleaving was in place).
So this is what is interesting. When I moved ISP (I'm on FTTC, and I moved from another FTTC provider), I got a new number. When I initially checked the number on the DSL checker, it was showing ADSL2+ speeds of upto 6 Mbps or something; I think the range may have been 4.5-11 Mbps or something, but I am not sure (but the estimate and range is about what I'd expect). I checked about a week later and the estimates had an uplift; at present it is showing up to 11.5 Mbps with a range between 6.5 and 19 Mbps.
Does anyone have an idea to what could have caused the uplift to such estimates? The line should only be capable of attaining speeds at the lower end of the estimate range. I am not aware of any new cabling, and the other line (plus nearby properties) does not fall into this range at all. Maybe they just got their information wrong on the database?