It's not just depth/distance from the piston that matters. You'll notice in the two plugs on the right that the electrode is more exposed.
Found this:
http://www.ngk.com.au/sparkplug_tech.phpIt explains many of the common aspects of spark plugs including projection:
"A projected spark plug protrudes into the combustion chamber further and provides higher ignitability and improved performance. Simply this can be understood by representing the combustion chamber as a circle. A projected spark plug in effect produces a spark in the middle of the circle (or combustion chamber) allowing for an even flame spread. Conversely, a non-projected spark plug means the flame spread is not even."
It also explains the precious metal electrode nicely. Since it has a higher melting point it can be narrower and again, gives you a cleaner, more precise spark resulting in better performance, whiter whites, britghter colors, and world peace
I think at least one user noted better performance (notably more stable idle IIRC) with a precious metal plug.
Scott