When you're asked a question, you give more information to a trained examiner than simply what's in your answer. Your tone of voice, degree of eye contact, your body language and facial expressions, how long you take to give an answer: all these and more communicate information. El Al famously extensively train their security personnel to pick up on and correlate all this other data; various customs and border services around the world try, and to varying degrees succeed.
So don't get hung up on the idea that you're being asked a question just because they want to know the answer. Sure, what you say will be of interest to the border guard, but what else you communicate while answering will be of interest also. They're trying to identify people using fake credentials, people entering with the intention of committing crimes, people impersonating other people, and the like, and asking a few simple factual questions gives such people a chance to give themselves away.
You do, as you point out, have a qualified treaty right to enter Bulgaria as an EU Citizen. So you could perfectly happily give anodyne and content-free answers if you wanted: I'm just coming to see Bulgaria, I don't have any particular plans, I don't really know where I'll be staying, etc. You may find your questioning is more prolonged than those who are prepared to answer in detail, but at the end of the day you are an EU Citizen, and your bona fides are in order, so they need a good reason not to admit you, and you're not obliged to provide them with one.