That is definitely a common experience. I still feel like I did terribly, even though I've scored 750+ a few times. My fiancee (760) felt the same - she was expecting something in the 500s! In fact, she almost cancelled her score!!!!
The reason makes sense. The GMAT is adaptive, so no matter how well you're doing, it keeps giving you harder and harder questions. So, even if you're in the 700s, you're going to be getting questions that are hard for you. Remember that it's okay to get a lot of questions wrong as long as you're in that upper level material; getting one wrong every 3 or 4 questions certainly won't hurt your score much.
I'll even add that people who think they did well, most likely did not! If you think the test was easy, you probably missed a lot of issues/wrinkles in problems, thereby falling for tricks and traps set by the test makers.
Hope this helps!
Jim S. | GMAT Instructor | Veritas Prep