As Lily.Petal said, you'll see this behaviour on any class with actual stances - Shadow Priests, Rogues, Druids, and Warriors, specifically. It's 7-10 - "stanceless" uses Bar 1, but Warriors are always in a stance, so when you place an ability on "bar 1", it's actually paging to Page 7 (aka Bar 7), and is active there.
You should be able to turn off automatic paging in the configuration for your Bar 1, however - I know it's possible with Dominos, and I know Bartender has the same options, I just don't know WHERE the options are in BT.