Youtuube voting system

I am still wondering how the YT voting system works. I see that newer comments with just few "vote ups" are shown in "Highest Rated Comments" section. If a video gets lots of views (or comments?), there are always older ones with much more vote ups that are not there.

Does somebody knows how the algorithm could work? I suppose newer comments have higher priority and less vote ups are needed to appear in highest rated comments. Or older comments are simply ignored by the algorithm?

thanx