I have some ideas / thoughts but it is theory only! the coking is only on the exhaust side as it engine might be mapped a little rich this means that the fuel does not quite burn 100% so you get carbon deposits. Also as engine goes through the 4 stroke cycle (suck, squeeze, bang & blow) it will be pulling in a cool (ish) mix of fuel air and may be N2O this especially as liquid fuel is vaporising and the N2O will be if used too is very good as keeping the inlet valve(s) cold and minimising the heat stress on that part of the engine head, unfortunately on the exhaust side of the head the valve(s) have a super hot mix gasses rushing past them as they exit the cylinder this will maximise heat stress in this area. This is why bronze valve guides and sodium filled valves are used in some highly tuned engines as they aid in the transfer of heat away from this part of the head. Ford actually at one point had a water cooled valve guide system for F1 to aid in the transfer of heat out of the head to stop them failing!

I had this done on my CVH engine and it makes a huge difference! unfortunately the first head was cracked by then! Though it was used as we cut it to bits to work out how to do the mods we wanted!