Highly doubt it's "fake". Dell gets their junk early all the time.
Regardless of when they got it 580s don't have Ellesmere chips.
If you put a BMW hood ornament on a Kia it's still a Kia.
Edit: I did some reading out of curiosity. The pixel and texture fillrates are the specs on 480s as well, not 580s. Other than the name, all fields that GPU-Z would report differently between a 480 and a 580 show those cards to be 480s. Looks like Dell has been naughty indeed.
https://en.m.wikipedia.org/wiki/AMD_Radeon_400_serieshttps://en.m.wikipedia.org/wiki/AMD_Radeon_500_series