What I'm meaning by the "gap" and since we don't know its merely a theory. If you imagine DNA like a ladder with two sides one side having the info for the spider and the other the WT. If the spider gene is missing half of a rung that is responsible for motor function. Now lets just say the normal gene has its half of the info needed to make proper motor function happen but since the spider side doesn't it wobbles. Now lets say you add pastel in the mix again just for an example. You still have the WT side that makes motor function happen but now you also have the pastel gene in the mix that might match complete the gene needed for motor function.

It's just a theory no one really knows how or why they do the wobble thing but its fun to think about the ways it might happen.