This doenst make any sense

ok, so after installing my new hard drive…i put about 8.5 gigs into it…its still got like 170 something free…when i right click on it in my computer…it says that 12.2 gigs have been used…so i double click on the drive and it shows me the two folders i have made…i hover my mouse over the first on and it says 8.24 gigs, and then i hover my mouse over the second one and it says 325 MB. so how does that add up to 12.2 gigs?