ISPs provide you with a speed range, which is their estimated range. These figures are based on the BT's DSL Checker's estimates, although they may be slightly different depending on whether an ISP wishes to be conservative on the estimates or not (so some may have a lower range than others).
A minimum guarantee, however, is usually based on the Downstream Handback Threshold (which in your case, as you can see, is 56 Mbps) and not the range itself. If the line speed falls below the threshold then it is determined to be underperforming. Openreach will investigate if prompted by the ISP after all the usual tests are done, and if no improvements can be made then the ISP will generally allow you to exit contract without a fee.
Just to note, I think the BT website, when ordering broadband, shows estimates and guarantees based on address rather than telephone number. So there may be a slight discrepancy there. Either way, you're above the guarantee and below the estimates... but you're also out of contract, so you could just leave if you were not satisfied but you wouldn't get any faster on another ISP and you should probably wait until this "fault" has been investigated fully if you wanted to leave.
Of course, it goes deeper than this, but this is slightly more in layman's terms.