Why am I getting shocked in a very specific way when petting my cat?

It's a dry, windy day. I had my left hand stationary on my cat's back and was petting him with my right hand. I noticed when I removed my right hand from contact with his fur, I got tiny shocks in the middle finger of my left hand, but only when removing my right hand and only in my left middle finger. (He didn't seem to notice any shocks.)

Why is this very specific type of electric shock occurring?