Luminance/contrast - Displays have adjustments in luminance and contrast to account for ambient lighting, which can vary widely (e.g., from the glare of bright clouds to moonless night approaches to minimally lit fields).
Boresight - Aircraft HUD components are very accurately aligned with the aircraft's three axes - a process called boresighting - so that displayed data conforms to reality typically with an accuracy of ±7.0 milliradians (±24 MOA), and may vary across the HUD's FOV.