A Reel fills the screen while it plays in the Reels tab. In the feed, though, Instagram shows the same video in a 4:5 crop – a little is cut off at the top and bottom. Anything important therefore belongs in the middle of the frame.
A landscape video becomes a Reel when you pick a 9:16 crop that follows the subject, or keep the whole picture with a filled border. For the feed, a separate 4:5 version at 1080 × 1350 pixels is worth it.