Broadband Availability checker reports my line slower than neighbours

[#1]

I have just run the BT broadband checker and out of curiosity tried some of my close neighbours' numbers as well as my own with results as follows:
Neighbours: ADSL Max broadband line speed of 7.5Mbps and ADSL2+ broadband line speed of 15Mbps
Mine: ADSL Max broadband line speed of 7Mbps and ADSL2+ broadband line speed of 13Mbps
I live in a block of flats and all the lines enter the flats via a distribution box.
Now it is not a huge difference but I am intrigued as to why it exists at all. I don't know whether any of the neighbours concerned have broadband but would that make any difference. Does the difference indicate some deficiency in my line?
Any ideas?
Thanks
Dave

[#2]

How long you had your line? My line comes In via a distribution box aswell ( I live In a block of flats at the moment too) and I get really good results.

[#3]

The line has been there for 20+ years.
Dave

[#4]

Hi daveplus,
not sure why it would show that difference, but I suppose the first thing to remember is that it is an estimate. I can't remember where i read it but there had been mention of discrepancies with the availability checker and noises about changing the way it estimates to be more accurate.
Jojo