I'm puzzled. When I check two houses which are next door to each other, the one which is further away (albeit a full 10 yards) gets an almost 50% higher estimated maximum speed on the BT Broadband checker, 3.5Mbps vs. 5Mbps.
The houses are of the same age and both have broadband connections, although I've not tested the speeds I do know that the one with the higher estimate does have higher speeds than the other, so the estimate seems to be accurate.
As they have broadband they won't be estimated speeds, they will be actual speeds. Perhaps one house has the bell wire connected, the other doesn't. Perhaps one has a faulty filter. Or as Sandgrounder suggests the lines may take a different route.
I think the lower speed it a more accurate representation of the area. I live a further 100 yards down the road (closer to the exchange), on a side road, and mine is 3.5Mbps. The speed of the 'faster' house just seemed to be an exception, hence my question as to what this may be.