Every preset here targets a fixed aspect ratio — a YouTube thumbnail is wider than it is tall, an Instagram Story is much taller than it is wide. Since your source photo rarely matches that ratio exactly, the tool scales it up or down until it completely covers the target frame, then crops away whatever overflows equally from both sides (or top and bottom), keeping the result centered.
That's the same "cover" behavior used across the web for responsive images and video thumbnails — it guarantees the frame is always filled with no empty bars, at the cost of trimming the edges of a photo that doesn't match the target proportions.