Quite simple, aniki. The feeds were ordered by hot, new, or top.
New was ORDER BY date DESC. Top was ORDER BY upvotes DESC. And hot was a slightly more complicated order that used a mixture of upvotes and time.
You can easily verify this by opening 2 different browsers in incognito mode and go to the old reddit frontpage - I get the same results in either. Again - I can't account for the new reddit site because I never used it for more than a few minutes, but that's definitely how they old one worked and still seems to.