Range A is classed as a good line while Range B is what they call an impacted line which means it may have a bridge tap occurs when one cable is spliced into another to break out cable pairs. This can cause slower speeds. Unfortunatly you don't know which Range you are on until you get connected.
This means that your true estimate is between the lower Range B and highest Range A figures which in your case is between 41.3Mbps and 79.6Mbps. It does not mean the speed will fluctuate between them it means your speed will be some where in-between them.