The problem is very simple : try this for example 

levenshtein("debugging","debuging") returns 0 instead of 1
levenshtein("debbbug","debug") returns 1 instead of 2..
etc

and

levenshtein("ddebug","debug") returns 0
etc

but (the exception!)

levenshtein("debugg","debug") returns 1 (coool)

My english is pretty poor so no long description, i think u have understood the core of the problem :)
I think this is a problem in the source code of the lenvenshtein function (algorithmic) so no need for other information (i hope so).

Have a nice day !

Francois