This is simply because the bit-width of the image as a whole is greater than other bands.
It is unclear if this is because
it has greater resolution, e.g. more pixels
it has the same number of pixels but those pixels each have multiple values coinciding with one another (seems unlikely, the geotiff could have just been written with multiple bands)
each pixel is a uint32 instead of uint16 (also seems unlikely, geotiff format supports both of those types, plus many others)
This is simply because the bit-width of the image as a whole is greater than other bands.
It is unclear if this is because