Answer:
First, if we compare 645 and 738, the difference is significant, because those number have different units, different tenths and different hundreds.
However, if we compara 645 and 649, the difference isn't significant, because they have the same hundreds, the same tenths, but different units.
So, in terms of comparing numbers as such, we can tell that the difference in each comparison is about units, tenth and hundreds difference.
From this different comparison, the difference operation (subtraction) is also different, because it's not the same to subtract 645 from 738 than from 649, the difference is higher.