Why my Apple Watch series 3 (GPS) has 4.8 GB of capacity, and it has to be 8GB ??

Quite a few GB's are used for the operating system not to mention the Apps or songs, photos, etc.

Watch Series 3

GPS Model: 8 GB

GPS + Cellular Model: 16 GB

You have to subtract watchOS, builtin apps and stuff you sync over from your iPhone from the capacity.

8 GB is the capacity without anything not even watchOS.

I have the same situation. I understand some of the 8gb are used for the OS, but I've seen in several sites that I should have a capacity of 5.8gb, not 4.8gb.