Now, now, I'm sure the first guy was "enhancing" his own specs a little bit, too. We all do it.
That said, there are some arguably similar drives listed at 500 GB capacity on ebay. But it's not the size of the disk, it's the gesticulation of the allocation.
I have several 500gb hdd's. I've never seen one labled 512 gb. Only micro sd cards and SSD's. If they did exist, I'd suspect they were either IDE or SAS.
I feel like for HDDs it is. In general it is not. The even weirder thing is that HDD sizes are expressed in decimal gigabytes to inflate the numbers. A 500 GB drive will show as around 480 in some operating systems.