GIF counts delays in hundredths of a second, so 20, 25 and 50 land exactly. The rest are spread to keep the length right.
256 is the most a GIF can hold. Fewer colors means a smaller file.
Off keeps text and flat color crisp. Dithering hides banding in photos and gradients.
One palette keeps colors steady and the file small. Per frame is sharper when the scene changes a lot.
Faster takes fewer frames from the same stretch of video.