I used to have this problem sporadically on my 105/Ultegra setup on my old bike. My DH says this happens to him about 1/3 of the time. I'm definitely not a component guru and know very little about the mechanics of the bike, but I know it's not a "half shift". It's something to do with the position of the chainring when you shift. Your LBS might be able to tune it up for you. I don't think it's a bad thing, but I also don't think it should happen all the time.
On my new bike with all Ultegra, it doesn't happen at all... one tap and I move into the large or small ring.