The difference between the larger result and the smaller result was 10.
Suppose the 4 digits numbers are abcd and pqrs
Here, 1st Number is = 1000*a + 100*b + 10*c + d
and 2nd Number is = 1000*ap + 100*q + 10*r + s
Then, since Kent made an error if 1 in tens columns and Harold added it correct, so the difference will be of 10 points.