Also if it revs a little bit, technically you are taking a little load off of the engine when you are putting it back into neutral and not into gear with a load on it. A slight bump in RMPs isn't too surprising, then the engine just idles back down to normal.
It's like when working on cars when you have to adjust the idle, you want to have it at a certain rpm in neutral, but when you put it in gear it drops some from the load. If you have a vacuum leak or bad valve or something it will drop too much and not run right or kill the engine. In my experience, an engine will always idle slightly higher in neutral, at least temporarily until the computer adjusts it down again.