This happens whenever you tap on a pokemon before c-day begins. Your client (the app copy of Pokemon GO you own on your particular phone) "remembers" what a Pokemon is before it gets substituted after an event begins. This happens because C-Days swaps spawns already in the overworld for the featured species the moment it begins.
So you probably clicked on this Voltorb before the event began and then, after C-day begun, it got substituted in the overworld with a Chikorita that happened to be shiny, but your game still recognized it as a Voltorb because you "revealed" its identity prior and it became what it truly was according to your app. There are reports of this every C-Day.
That's why this site recommends not even tapping on a Pokemon at least 30 minutes before the C-day starts in their guides.
Furthermore, if you catch all Pokemon in your nearby-cluster a few minutes before C-day begins, you will have a lower rate of first-minute appearances of the Pokemon featured on that C-day because you have already caught them, just on their original spawn forms.
