Because each hard drive has areas that are not used directly by the user and there is no exact specification for it.
An example is the boot area for starting the operating system.For example, few users will know that Windows creates a 100MB partition at the beginning of the hard drive as part of the installation. This is unknown to most users because it is not displayed in the operating system. If you look at the hard drive with Linux, you can see it immediately, for example. Microsoft places files here that are responsible for booting and the operating system core.
Another reason is that, for example, every file system also needs and uses an index.You can imagine the principle as the linked table of contents in a PDF eBook.
This table of contents also needs space.However, this varies depending on the file system. The same applies to the boot area. For example, legacy BIOS and an MBR partition table require less memory than the newer GBT standard, which includes a one-time ID. Which is not the case with Legacy BIOS.
An example of the differences in the file system index is that file names in DOS could not have more than 8+3 characters and no special characters.NTFS, for example, allows not only special characters, but also much longer file names. Which logically needs more storage space. In addition to different block sizes. This space is RESERVED according to file system and block size! That’s why you never see the full storage space. Without formatting and without a file system, the hard drive would be more or less unusable. And with is pre-reserved and you won’t see the full storage space anymore.
The question here is how the hard drive manufacturer should know which file system and which partition table the user uses at the end!That is unlikely to be possible. In this respect, the manufacturer has no choice but to specify the raw storage space. As an alternative, one now sees at least ESTIMATED information, how much space will remain free approximately after deductions.
Apart from the fact that many 08/15 users probably do not know to this day that originally the computer kilo was not equal to 1000, but 1024 AND probably still not known to most standard users that gb and GiB exist that this difference is still common. to make clear.
In passing, there are also areas for errors in hard drives.For example, if the hard drive has bad sectors, they are disabled and no longer used.
However, the technology is designed in such a way that the average user does not notice this, or only when it is already too late.And the fact that hard drives are broken on arrival or after a few weeks, even new ones, is unfortunately not a rarity today, but almost the rule.
By the way, just to say that, data also consumes different amount of storage space depending on the hard drive.Depending on the operating system, you can see e.g. in the properties of the files (right click, properties or file properties, or similar… depending on the file manager) 2 sizes. The file size and the amount used in memory. The first should normally be the same even for different hard drives. Second is different depending on the hard drive and file system. So if you hang up at the smallest MB, you have a problem here anyway. But with himself. Not with the hard drive.