Help for the 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.
It may also help to just add zeros to the decimal numbers. For example