I'm not disagreeing with the method you're using - all I'm saying is that if you're "scrolling down the list to see what's in the messages", then yes, that involves reading the messages. Scrolling while selecting, which is what you're doing, will refresh the message pane (the right-hand pane), and Apple Mail has no way of knowing whether your eyes are actually looking at that pane. (Removing the message pane removes the doubt.)
If you don't want to read the actual message, and the 2-line preview in the central pane is enough to help you decide whether to read it or not, use the two-finger scroll. This will not refresh the message pane, and if the 2-line preview interests you, you can then click on the message to select and read it.
Don't forget that you can use flags, instead of read/unread status, to highlight messages that are important enough to be addressed later. Right-click a message preview to select a flag, or shift-cmd-L to assign a red flag - you can use the Keyboard preference panel in System Preferences to assign shortcuts to the other flag options.
There's now a "Flagged" mailbox by default in Apple Mail, though you can set up any smart mailbox to do a similar operation.
Matt