Posts have an exact time stamp including seconds, except that only shows the hour and minute the post was submitted. But let's say you submit a post at 13:57:03, than there are still 57 seconds left for someone else posting a post, which has the same time stamp (13:57), but their post will appear after your's, or on top depending on the browsing settings. All submitted posts are organized by time, so if your post is below, it just means you submitted the post earlier than someone else.
Let me know, if this was unclear.