In Apple Mail, why are all your email accounts listed in the sidebar in small caps, and then all listed again down the sidebar in CAPS?

What is the difference, and why does Apple Mail have your email address accounts listed twice in the sidebar? It seems that the all CAPS email accounts towards the bottom of the sidebar have folders in them for Sent, SPAM, etc., when you click on the disclosure triangle to the left of the email address. Well, anyway, what is the point of the this? What is the difference between the two listings of your email accounts, and can this be changed in Preferences? Thanks.