In a decimal system, books are filed digit by digit, not by whole number. Look at this example of numbers in their correct order:
If you're confused, it might help to think of Dewey numbers as money. For example, if you have $120.20, this is more than $120.02. And $979.50 is more than $979.1799. Just think of every book in the library as a sum of money from $000.0000 to $999.9999.