Probably because lukeprog has higher status. It's surprisingly hard to overcome innate irrational political voting patterns, even on a website like this one.

I think a comment gets pretty fair karma evaluation here when it contains some non-trivial idea which forces the readers to think about it (although comments of established authors will still fare better because much more people will read them). On the other hand, trivial remarks like the grand-parent or the discussed lukeprog's comment have karma mostly determined by their author's status.