But surely that's down to timing, I'm only answering to individual post as they come up on my screen.
That's because your replying to posts as you scroll down the screen and browse through the pages of history. Proper etiquette is to first read a thread to the latest comment and then quote the posts you wish to reply to. This allows you to quote them all in one single post.
That is how most people do it when replying to multiple people.
But like I said, don't worry about it. There is no need to discuss this unless some loony comes on and goes overboard with how they post.