[freefal]

It could be that 825GB is the typical marketing GB (i.e., 1000^3) and the 667GB is the OS reported GB (i.e., 1024^3). That difference only reduces the 825 "marketing" GB to 768 "real" GB. So still a 100GB of OS/system files, which seems large, as you say.

[akhosravian]

giga- (along with kilo-, mega-, peta- and every other prefix we've ever used with bytes) in any other context is based on powers of 10.

Just because computers are based on binary doesn't mean it was ever a good idea to redefine the meaning of those prefixes.