Gmail's Priority Inbox is an superior version of the regular inbox, with customizable sections for power users. For some people, Priority Inbox looked threatening, so that's most likely the reason why Gmail tests a new version of the regular inbox that integrates Priority Inbox and three lightweight versions of the Priority Inbox.
"Try out all the new inbox styles to see what fits you best. You can always switch back if you change your mind," informs a Gmail promotional box. There are five inbox types: classic, important first, unread first, starred first and Priority Inbox. The three new inbox types could easily be obtained by customizing Priority Inbox, but it's much easier to switch flanked by them. It's the first time when Google uses tabs in Gmail interface if you keep out Gmail's settings page.