I was under the impression only FRAGTRAP could climb stairs but in the mission Home Sweet Home you follow a claptrap down the stairs to open Jack’s Office.
Continuity is a frequent casualty of video game story-telling. Don’t worry about it.
Maybe he can go down but not up. They did call it a Stair CLIMBING Wheel. Easy to go down stairs on a Segway but not up.
Even easier to go down stairs with a Segway - especially if you’re not watching where you’re going!