From my phone:
From Windows 7 SP1 x64 Ultimate:
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Your phone is showing you the total amount of storage inside your device. This includes a bunch of partitions containing the operating system, radio image, preloaded apps, bootloader, recovery, etc. These partitions are read-only, and many aren't even accessible within the Android OS, so they aren't exposed when connected to Windows—there wouldn't be any way to say "actually there's more space but you can't use it." What Windows sees is the user-accessible space it can read from and write to, which is smaller than the total storage in the device.