Technically manufacturers should use GiB not GB when marketting and packaging their drives, but they don't/won't, and use Gigabyte which they know practically everyone uses incorrectly.
Note that all this is just so much technical drivel, your actual space available is the same on both, it's just that from a marketing perspective the given size will be correct on Mac as they have a common understanding of what a Gigabyte is, and incorrect on Windows which uses the commonly misused Gibibyte structure on it's filesystems whilst users incorrectly assume it is measures in Gigabytes. Since Mac OS X also switched to this 'proper' definition for storage units such as Gigabytes as of Snow Leopard, then the size of the drive as mentioned on the box is more likely to be accurate when formatted for a Mac Partition than for a Windows Partition where your usable space seems 'lower'. It is interesting to note that Hard Drive Manufacturers use the correct meaning of 'Gigabytes' when referring to hard drives, and not what most people think of as a GB which is actually a Gibibyte:
