The computer isn't wrong. The sizes of a [Kilo/Mega/Giga/Tera/...]Byte were defined as 1024 Bytes/Kilobytes/Megabytes/Gigabytes back when computers were first invented. Much, MUCH later, the terms were redefined by HardDisk manufacturers so they could get to a certain 'magic size' faster. A bit after that, the HD manufacturers got sued for misrepresenting the size of their drives, so now we see their definition printed on the box. At some point after that, the SI standards body redefined the size of a Kilo/Mega/Giga/Tera/...Byte to match what the HD manufacturers said it was.
Technically the definition has been changed, but the only people who actually use the new definition are the storage-device manufacturers.