I guess I can't post links yet (or they were caught in a filter or something), but you can take my word for it.

I ran GPU-Z when I first installed the card just to make sure there were no surprises, so I knew 6144 MB was the total amount. GPU Shark and GPU-Z both show the same thing, no problems there.
 
Running the FFXV benchmark a few times always displayed the same amount of VRAM. I thought it might change if other processes were running in the background, but it didn't, and the benchmark settings were all the same for the results I compared with, so that's what got me wondering about it. But it makes sense that hardware and OS would affect the amount that was shown, I should've realised. Thanks for the help, guys.

You know I completely ignored the tools menu earlier, thanks for pointing that out. The detailed view I did see, but there was nothing that stood out. This is the data:
 

GPU Shark v0.11.1
(C)2013 Geeks3D - www.geeks3d.com
----------------------------------------
- Elapsed time: 00:04:27
- Windows 7 64-bit build 7601 [Service Pack 1]
- OpenGL info:
 - GL_VERSION: 4.6.0 NVIDIA 390.65 (# ext: 405)
 - GL_RENDERER: GeForce GTX 1060 6GB/PCIe/SSE2
 - GL memory - total:6144MB, usage:939MB
  GPU 1 - NVIDIA GeForce GTX 1060 6GB
- GPU: GP106 (rev: A1)
- Bus ID: 5
- Device ID: 10DE-1C03
- Subvendor: EVGA (3842-6163)
- Driver version: R390.65
- NV driver branch: r390_63-2
- Bios version: 86.06.63.00.60
- GPU memory size: 6143MB
- GPU memory vendor: Hynix
- GPU memory type: 192-bit GDDR5
- GPU memory location: GPU dedicated
- GPU temp: 36.0°C (min:36.0°C - max:37.0°C)
- Fan speed: 0.0% / 0.0 RPM
- GPU cores: 1280
- GPU ROPs: 48
- GPU Texture units: 80
- TDP: 120 Watts
- Current clock speeds:
  - Core: 139.0MHz
  - Texture fillrate: 11.1 GTexel/sec
  - Pixel fillrate: 6.7 GPixel/sec
  - Mem: 405.0MHz
  - Mem bandwidth: 19.4GB/s
- Base clock speeds:
  - GPU core: 1607.0MHz
  - Texture fillrate: 128.6 GTexel/s
  - Pixel fillrate: 77.1 GPixel/s
  - Mem: 4004.0MHz
  - Mem bandwidth: 192.2GB/s
- Boost clock speeds:
  - GPU core: 1835.0MHz
  - Texture fillrate: 146.8 GTexel/s
  - Pixel fillrate: 88.1 GPixel/s
  - Mem: 4004.0MHz
  - Mem bandwidth: 192.2GB/s
- Clock speeds offsets:
  - GPU core: 0.0MHz
  - GPU voltage: 0.000V
  - Mem: 0.0MHz
- GPU power:
  - Current power: 7.4% TDP
  - Min power: 50.0% TDP
  - Max power: 116.7% TDP
  - Default power: 100.0% TDP
  - Power target: 100.0% TDP
- GPU and memory usage:
  - GPU: 10.0%, max: 33.0%
  - GPU memory: 15.3%
  - GPU memory controller: 7.0%
- Limiting policies (NVIDIA):
  - no limitation
- Current active 3D applications:
  --> dwm.exe (PID: 1900)
  --> setpoint.exe (PID: 2596)
  --> firefox.exe (PID: 5240)
  --> firefox.exe (PID: 5472)
  --> firefox.exe (PID: 1256)
  --> GPUShark.exe (PID: 5036)
  --> firefox.exe (PID: 5872)
  --> firefox.exe (PID: 4116)
  --> firefox.exe (PID: 6604)

But the Memory Info displays something I've never heard of before - "Eviction count: 11", "Evicted memory: 110MB" - should I be concerned or not?
 

Alrighty, thanks again!