What could be the reason for the following problem?

The hard drive is usually a type of hardware that you can install on your computer to have more space. You cannot expect that if you get a 250GB hard drive, you will have 250GB space available. There are different things that can affect that such as the formatting tables that are available.

The free space will be usually less than 250 GB. There are instances when hard drives already come with pre-installed apps and software so you can already expect that you will not have a 250GB amount of space available once you get your hard drive installed.

Take note that hard drives usually have a certain amount of space allocated to ensure that the apps and other software that you have already installed in your computer will work properly.