EDIT: It seems I have a misunderstanding about んじゃない. I thought it was meant to add "isn't it" to the end of a question. e.g. 田中さんはケーキを食べるんじゃない "田中 eats cake doesn't he?.
According to Tsuyoshi Ito's answer below it seems it can also be used to negate a sentence. I'm not sure what this means. Does it become "田中 doesn't eat cake does he?" or just "田中 doesn't eat cake". If the latter, how does it differ from 田中さんはケーキを食べない.
In summary, how do I know which way to interpret んじゃない?
I've looked at all the related questions on this site but none of them are clearing up my confusion.
ORIGINAL: I'm having trouble making any sense from this sentence:
After all, when you say you're ordinary, you're ordinary, so it makes you special doesn't it?
I don't really know how to interpret じゃないもん. Is that making my translation go wrong or is this sentence really as silly as my translation?