1000 and 1024 are both in use. The misunderstandings that follow from notation.
Storage manufacturers count a gigabyte as a billion bytes, while operating systems have long multiplied by 1024. A device labelled 500GB therefore shows as around 465GB when connected. Nothing went missing; the counting differs.
Computers work in binary, so powers of two are natural units. Two to the tenth is 1024, close enough to 1000 that early usage borrowed the kilo prefix. A standard later introduced separate names for the 1024 basis to remove the confusion, but in practice both remain mixed.
Internet speed is usually written in megabits per second while file size is in megabytes. A byte is eight bits, so they differ by eight. That is why a 100Mbps line downloading at around 12MB per second is normal. Feeling that speed falls short of what was advertised often comes from confusing these units.
Beyond the notation gap, usable capacity is smaller still. The file system's own structures, a recovery partition and pre-installed software all occupy space. That is why a new device is not empty from the start.
Knowing rough magnitudes makes planning easier than memorising numbers. A photograph is a few megabytes, a song a few megabytes, and high-quality video tens to hundreds of megabytes per minute. That much feel is enough for most decisions about backups or transfer times.
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