It is not planar. The original sketch is garbage at a slight angle to horizontal and vertical. Where did you get this sketch geometry? AutoCAD?
The revolve results in conic faces since the line that creates that face isn't perpendicular the axis of revolution.
I recommend you read this document http://home.pct.edu/~jmather/skillsusa%20university.pdf
I recommend you dimension your sketches - in which case the problem becomes rather obvious.