Explanation:
now let's think about this with sine concentration.
what is bigger - 4.3 or 4.39 ? 4.39, of course.
and 7.1 and 7.09 ? 7.1, of course.
imagine the numbers you compare without decimal point (but you need to give them the same number of digits, so you have to fill up with 0s) :
the most positions after the decimal point are here 2. and in total the most defining positions are here 3.
so, we write every number without decimal point but with 3 defining positions :
430, 439, 403, 710, 709
now it is easy to sort them, right ?
710, 709, 439, 430, 403
and in the same way the original numbers are sorted :
7.1, 7.09, 4.39, 4.3, 4.03
add on :
what if there were also 0.9 and 0.12 as numbers ?
they would be in our approach to show 3 defining positions : 090 and 012 = 90 and 12.