There is a logic to it but I wouldn't say it's the right logic...
The original post is shown at the top of the thread, then the replies underneath with the earliest one at the bottom. Each new reply to the original post is addedd above previous replies (but still underneath the original post). Replies to replies work on the same basis but because they're indented under the reply they're replying to then the dates and times in the right hand column can look like they're all over the place.
God, that explanation sounds even more complicated than the threads themselves!
The 'Flat' version is one of the options shown near the top of the page after you have clicked on a topic of interest. Not some arcane term only understood by introspective spotty teenage computer fanciers.
Click on the word flat where it says PREVIOUS INDEX NEXT FLAT and the word THREADED is in grey.