If you watch the video on p1 you'll see that depending on viewing angle the visible gaps vary quite significantly even though the aircraft aren't actually manoeuvering relative to each other - whilst they certainly fly very close formation, I doubt very much that it's actually 18" apart - but to be perfectly honest I don't really care that much <g> Whilst appreciating their skill, I tend to agree with Mr Bernoulli about the display, my favourites remain the Red Arrows from around 1970.
Dave