You did indeed get 500 GB. Just not the same format that computers use. When a retailer says "500 GB" they mean 500,000,000,000 bytes. Computers calculate storage differently.
8 bits = 1 byte
1024 bytes = 1 kilobyte (kB)
1024 kilobytes = 1 megabyte (MB)
1024 megabytes = 1 gigabyte (GB)
So if we take 500,000,000,000 and divide that by 1024, we get 488,281,250 kilobytes. Divide by 1024 again and we get 476,837.16 megabytes. Divide by 1024 yet again, and we get 465.66 gigabytes.
So yes, your hard drive is "500 gigabytes". If you go to My Computer (assuming Windows here), right click on your C drive and click Properties, where it says "Capacity", It will have very close to "500,000,000,000" written, with "456 GB" off to the right.