Does it say 1.5GB is used, and that is %100? Or does it say that 2GB is used when in actuality only 1.5GB is?
If it is the former, what format is the thing? Is it NTFS? FAT? FAT32? EXFAT? Some other format? You may have corruption on your drive, and some formats will just mark those sections as unusable and that will decrease the capacity of your drive... Or it could be extraneous overhead in the format specification that is taking up a lot of room...
If it is the latter, it is very possible that, as twocows mentioned, that you just have hidden files taking up space on there, like maybe autobackup or recover files for example. Or maybe files that came on the drive when you bought it.