I agree with OP, though I'd have left a tip of exactly 15%, on the dot, if otherwise the service was good. You still got service - you just also almost got screwed.
To anyone who doesn't understand this, imagine you go to a restaurant with your wife/girlfriend. You order a king cut of prime rib for dinner ($30). Your girlfriend orders a shrimp dinner.($20) When the food comes, you cut your prime rib in half and give half to your girlfriend. Then, she gives you half of her shrimp.
When the bill comes, you see this:
$20, $20, $15, $15. So, instead of $50 total, it's $70 total. Then find out it's because your waiter determined that since you cut the prime rib in half, he should charge you for two petit cuts. And, since the girlfriend split the shrimp dinner in half, you should be charged with two lunch sized orders of shrimp.
This, in essence, is what happened to the OP. There's no way in the world I would return to that restaurant, and I would immediately post a negative Yelp review.