Do you mean: If my Jolteon defeats 2 Litwick, why does it's Special Attack increase by 1?
This is because you need 4 EVs of one stat to increase that stat by 1. And when you defeat a Litwick, it gives you 1 Special Attack EV. So normally, what would happen here, is that you would need to defeat 4 Litwick for Jolteon's Special Attack to increase by 1. But in this situation, it increased after 2.
This is strange, but I think I've figured out why it happened. Your Jolteon must have already had at least 2 Special Attack EVs from somewhere else, somehow. Then it would only take 2 more EVs to make 4 EVs, to increase your Jolteon's Special Attack by 1.
Or, it could have had the Pokévirus, or it could have been holding the Macho Brace or the Power Lens, which double all the Special Attack EVs you gain, making it so that you only needed to defeat 2 Litwick.
I hope I helped!