When you compare 649 and 645, you see that the first two digits are the same, and you compare until the last digit: in a way you need to do 3 comparisons, one for each digit (and after first and second you conclude that you still need to compare).
when you compare 645 and 738 you see immediately that the second number is bigger: the numbers differ already in the number of hundreds than they have, so you don't need to compare their tenths and one's, just the hundreds.
