Help, trying to test GPU but VRAM readings are different across softwares

Just got a RX 580 2048sp (for an old machine, i5 2400, PCI-e 2.0 and W7) and I have a few days to test it. Other than some crashes due to first experience with amd drivers/adrenalin (radeon wattman crashing, on fan profiling I think), most other tests seem fine (1/2 hour on furmark, furmark rog and heaven).

But the one thing I'm not getting is when trying to stress test the VRAM (8Gb in theory, and shown on GPU-Z) with the furmark rog '6.5Gb VRAM' test, so I can confirm it has 8Gb of funtioning VRAM, some programs show over 7Gb VRAM use while others show under 3 (image), no matter what I do (even running Heaven on parallel). So which one is true? or even what am I misinterpreting in some of those readings? Or are some of my other hardware bottlenecks holding the VRAM utilization back?

Is there a risk the VRAM firmware info is 'spoofed' or something (like what's done to fake SSD sizes) and shows 8 but is less, or some of the memory chips are worn-out/bricked from bios/???

Thanks!