I just don’t understand how it happens. Do posters literally read the original post and reply without reading ANY other posts ?
Yes and why shouldn't they, especially, as in this case, there's a question that they choose to answer?
Also, if the opening post is essentially a complaint, as this one is (it's also one that keeps recurring, though that's not your fault), posters often want to answer the complaint without reading on to see if everyone else is complaining too.
This might be because, like me, they find such opening posts tiresome and, to put it bluntly, unreasonable.
I also think it's unreasonable to expect people to accept that a thread has been 'sorted'. It hasn't been sorted if someone reading it for the first time has something to say. Why shouldn't they add their tuppenceworth? Those who think 'sorted' could just leave the thread alone and, you know, stop annoying themselves by reading it.