When comparing these two different sets of numbers the major difference is the number of places that you need to compare. In the case of the first pair, you would have to include the ones, tens and hundreds place when comparing, whereas with the second pair you would only need to compare the ones place.
This can be seen by using subtraction. You'll note that subtracting the first set gives you a 2 digit number and involves the process of borrowing (when you do it by hand).
738 - 645 = 93
The second set will give you a one digit number and involves no borrowing.
