The drive manufacturers like to make their drives appear in the best possible light, therefore they divide their bytes by a 1000 ie. 1000 bytes = 1 Mb and 1000 Mb = 1Gb!
However, the reality in the computing world is slightly different. In reality it takes 1024 bytes to = 1 Mb and 1024 Mb to = 1Gb, hence the discrepancy.
The size indicated in Properties, is a true reflection of the actual disc size.