Should it not consider all posts in the topic? Seems to make more sense to me.
Nah. If the OP gets a lot of merits, it's a good indication that the thread is worthwhile. Sometimes the OP of a thread can write complete crap but a subsequent reply can earn a lot of merit--I don't have an example of that, but I'm pretty sure I've seen it happen.
Anyway, in a case like that I don't think the thread ought to be listed as one of the top-merited ones, because it would be somewhat misleading.