So is the High figure more likely than the lowest ?
In the middle?
BT come up with the range by comparing your line with the behaviour of all "similar" lines. I take "similar lines" to mean lines with the same/similar attenuation between cabinet and DP.
Looking at this pool of lines, they then take the high point of the range to be the 80th percentile of the pool; the low point comes from the 20th percentile.
So 20% of lines are above the highest speed of the range, 20% are below the lowest speed, and 60% are inbetween. BT seem to accept fault reports when speeds are in the bottom 10%.
The variation across all these lines almost certainly comes from crosstalk, so you should expect your speed to start out high, in the range, and end up lower over time - but likely still within the range.
Note too that these are all line speeds (ie sync speeds). Download speeds will be lower.