It's a really, really good question.  It's an older blog post but I tackled this issue here (see second half).   

In a nutshell: "What Neutral means is that there is little-to-no forcing coming from either El Niño or La Niña in the tropical Pacific. You can think of a forcing as a push in a certain direction. El Niño and La Niña shove around the global atmospheric circulation, and the lack of either means this pushing motion is absent."