Plural function receives 3 arguments: number, single and plural form. There are 2 problems:
1. Some languages have more than 2 forms.
2. Some phrases with numerals are the same in english ("1 new" and "10 new").
2.1. And because of that Vanilla authors did not pluralize such phrases
(quick tip: search for "%s new", there are three occurrences, and all of them are not plural and some of them even not translatable [no T() call]).
There are solutions for all of these problems, but maybe something could be done with Plural function itself to make it work better.
Currently possible solutions:
1. For third plural form use some kind of suffix (e.g., "_many")and add it to original string.
2. Make Plural function check if single form is the same as plural, and if they are, add another suffix.