|If I had 12 ounces of fluid and only 10 ounce cups then it would require two cups to hold the 10 ounces of fluid.|
20 ounces of storage space to hold 12 ounces.
40 ounces would require 4 cups or 40 ounces of available storage space.
Computers work in a similar way depending on the allocation unit sizes used on PC when it was formatted.
Look at the properties of a 1kb .txt file where the file size would show 1kb but it would then also show how much storage space it requires due to the size of your allocation units.
It only effects the percentage of the last unit required.
a 1001 ounce file would require 101 10 ounce allocation units leaving that last cup with 9 ounces of unusable space.