My guess is that he didn't spend much time in classes, preferring to be a Weatherunderground grunt. I've taken my share of classes in the past ten years where I've even been impressed by the contributions of a number of black students. If one of them became President, I certainly wouldn't know it was one of those who impressed me.
My own experience from my undergraduate days is that the only people whose names I remember are the ones I was friendly with. I wouldn't remember even the names most of others who also received Math degrees in my class.
As for finishing two years in one, my sister got her degree from Brandeis in three years without credits from any other institution. And no one can say that Obama didn't get credit from some as yet unknown institution, say maybe in Pakistan.
Obama's professors and classmates, including former international politics professor Michael Baron and current MTV president Michael Wolf, confirm that he was a brilliant, standout student and that he was an active participant in seminars. Baron said he was one of the top one or two students in his class. Despite this, Obama continually declines requests to release his Columbia transcript.ML/NJ
The reference you link to, wikicu, appears to be largely taken from Obama's own autobiographies written later, or his campaign materials, which contain many statements whose historical accuracy is highly dubious. There is absolutely not one direct quotation in the wikicu article from a Columbia classmate recalling Obama's days as a student there.
The only evidence vaguely suggesting he was a student there was an article the campus publication, "The Sundial," written under Obama's byline.