I expect preview to show posts from the end of thread. Replying usually is done for last post in the thread.
The current behavior has to do with what hitting "Reply" at the bottom of the page means -- we somewhat arbitrarily decided it meant replying to the OP regardless of what page of the thread you're on. So the context for the OP is indeed the first five posts in the thread.
However, maybe this is not the most useful behavior. I think we do want to keep things sorted such that replies that are not to any specific post are to the OP (since they usually are for short threads), but perhaps we should add a url parameter to the reply button so that hitting reply at the bottom of a page shows the last few posts on that page?
Unless I misremember how I coded it, the (up to) 5 context posts shown depend on which post you reply to. So if you reply to the thread, you get shown the OP. If you reply to the last post in the thread, you get shown the last 5 posts.
We could just make the reply button at the bottom of the page mean you're replying to the last post on that page, showing the relevant context. Thoughts welcome on what context to grab in which case.