I'm new to Twitter and I use it in a web-browser on my phone.
When I open somebody's profile and go to "Replies" tab on their profile, I see the replies that this person gave to various tweets. It looks like this: an original tweet, and a reply below. There's also a grey vertical line on the left side, connecting the original tweet and the reply. The reply contains a mark "Replying to...". This is pretty clear to me.
But some posts in this "Replies" tab are not connected to any other tweets, are not linked to them with a grey line, and do not start with a "Replying to..." mark. They look just like a simple tweet, not a reply. What are these tweets? Why are they shown in "Replies" tab?