I am starting to notice that myself. I noticed that there are people who come to a post and upvote a comment and they don't even upvote the content itself. The comment that gets upvoted is usually something generic like "great post, keep up the good work".

I would think you would upvote the content if you are going to upvote reply comment. The only time I would think that you would upvote a comment without upvoting the original post is if you disagree with the original post and you think the comment a strong counter to the original post.