Why did fact checking site "politifact" rate michele bachman as a compulsive liar?

You answered your own question. She has only told the whole truth once out of 23 times. That sounds like a compulsive liar to me. To the folks who commented about Obama's results, he did much much better, with 212 out of 303 statements being true, mostly true, or half true. By that standard Bachman got 13% to Obama's 70%. More below.