Yes, that is very common. Your OS usually uses some of the missing storage and you can't see it. Your drive is fine and that is absolutely normal that 10-15 Gigs won't show up.
The manufacturer goes by the 1000 system: 1000MB = 1GB. All computers go by the 1024 system: 1024MB = 1GB. This is just a way that the manufacturer can trick the buyer.