Why are hdd's never completely empty when formatted?

[Phil2008]

Ive notice this on all hdd's Ive had over the years,, why is this?

 

[Phil2008]

no idea,maybe boot record/hidden partition? havnt really noticed to be honest

Nope Ive deleted all partitions before doing a full format on this drive. Also with other hdd's Ive had, Ive used tools that restores the drive to the factory default,(kill disk) but na, space was still used after a quick format

I thought it was a virus or something when I noticed this for the first time about 4-5yrs ago