It’s the difference between Terabyte and Tebibyte. Terabytes are base 10 but Tebibytes are base 2. So if you translate between the two you’re losing whatever factor of 2 is closest to your decimal number. For example 1 kilobyte = 1000 bytes or 10^3 bytes. 1 kibibyte =1024 bytes or 2^10 bytes. It’s just a way to advertise and seem like it’s larger when it’s not. It is frustrating.