I think if you took off the OS that it came with and anything else the factory and retailer put on it, you might get closer.
I thought it was common knowledge that all comps or devices that have hard drives are always less than advertised. For example, I recently bought an iPod touch 4G 8GB (yes, I know it's small but I'm not rich) and it only had 6.5GB of usable space. Normal, I thought. My new comp advertises 320GB HDD. On first use, it offered 271 or something like that. I thought that was normal.