If you click on the Received column the items are sorted by date, oldest first or newest first.
If you sort with the OLDEST first (at the top) then pressing Previous takes you to an EARLIER email not LATER.
Previous can mean earlier in date, but can also mean preceding, in this case the one before in the list.
Most people work through a list from top down to bottom.
So Previous takes you to the previous one in the list (upwards). Next takes you to the next one in the list (downwards).
This is irrespective if they are sorted in asceding date order or decending date order.