Top comments is selected so some comments may have been filtered out. What's the purpose of this on facebook?

I’m reading on my tablet so don’t know if that makes a difference.

I always click it to “most recent” so then I see all

I think it’s Facebook way to filter out low quality comments like personal chit-chat, attack or simply comments that don’t make much sense. Youtube is doing something similar too.

It is done to improve the delivery to your page, especially when there are dozens or hundreds of replies.

Gets rid of spam and pointless banter.