I don't think it's the word "ending" so much as the grammar. It sounds like you mean the word N was registered with a certain percentage of endings, whereas you

I don't think it's the word "ending" so much as the grammar. It sounds like you mean the word N was registered with a certain percentage of endings, whereas you say a certain percentage of endings were registered with the word N. It seems "domain ending" might be a somewhat common way to describe a TLD in lay terms. That said, I am a geek and a curmudgeon, so I still think writing out top-level domain makes the most sense in such a context.