This allowed you to put in absurdly large numbers. I had a 9.5Mb floppy drive when I first realized the number was arbitrary, but expected to be reasonably accurate given "typical use" (which was code for "not encrypted and not already compressed").

Of course when you used higher than realistic numbers, you'd just eventually hit the last compressed cluster and then your system puked while the software tried to figure out why it can't write the next sector.