Sometimes you only want to answer the opening post. It's not necessary to read the whole thread to do that.
Other times you start reading the thread and come across a post you want to answer, so you do that even though there may be fifty other posts between it and your reply. If you state what you're answering that shouldn't be a problem.
So, in answer to the OP, I'd say: "It depends..." but, yes, total disconnect can be a bit weird. It might just be that someone has misunderstood stuff though only got half a picture because thier concentration lapsed–mitigating circumstances.
I read about half a dozen posts and then said my bit. I don't think anyone can really complain about that.