Why does pulling the mixture to idle cutoff increase the RPM slightly?
Engine are usually run slightly richer than a stoichiometric mixture, to ensure reliable running and avoid overheating the pistons and valves. When you cut off the fuel, it goes from slightly too much fuel to none - and so it's briefly at a stoichiometric ratio.

The engine runs slightly faster at this ratio because it's not wasting energy heating up unburnt fuel. 

If you see more than a 50rpm rise, it was previously running further from stoichiometric than necessary (as determined by the engine manufacturer, and many years of experience), reducing power and wasting fuel.