On many LED screens, a technique known as ‘virtual pixel pitch' is used which shares LEDs between pixels, thus making the giant screen appear to be a higher resolution.  

So when someone talks about a 'virtual pixel pitch' of 10mm, the screen is likely to actually have a 20mm pitch, but use virtual pixel technology

It's important to be aware that a virtual pixel pitch does not make a huge amount of different.  

For instance, a 10mm ‘virtual' screen will never look nearly as good as an ‘actual' 10mm screen, 

it is always best to go for the smallest ‘actual' pixel pitch that you can afford.