This is not a bug in SDL Passolo, it is a problem in your code.
First I would also point you to the answer from Uwe Schwenk. There is a PslStatistics object that can instantly give you the word count for a complete translation list.
On the other side when are looping the complete translation list as shown above you will also count the words from the font strings. To get a correct result you have to check within your loop if translation list entries are read-only or hidden like the font string entries and then exclude them from your calculation.