simply because /media/disk has a mapping that will point back to all usb devices plugged in. Due to the PnP feature in linux. if you look at the last line that is where you will see the mapping. but you could have done mount /dev/sdb1 but then you would have had to specify the /media/disk afterwards.
moutnt /dev/sdb1 /media/disk
repo was just giving you a shorter alternative.