I feel like people either think I'm really smart or really stupid. It's like they know I must be different from them because I rarely speak in their presence, but they have to fill in the blank as to what that difference is. I've had people talk down to me like I'm a child and remind me of really obvious things that I already knew; and I've also had people act, like you said, as if I was really smart and just holding back all these crazy smart thoughts I was having because I was too lacking in social skills to voice them. I'm not very good at being objective about myself either, but I would guess that my level of intelligence is pretty close to average.
For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life." - John 3:16