'Extra thickness' is the effect of what is known as 'distended hu doubling'. This is caused by flattening of a die through overpressure during hubbing, which flattens the devices on the die, stretching them out of shape. Subsequent hubbings at correct pressure place digits where they should be, thus creating the doubling.
This is the most difficult type of hub doubling for novices to detect because there usually is no 'separation' in the doubling they are taught to look for. The chief 'determinant' in detecting class 6 hub doubling is simply that letters are fatter than they should be.