That's my NOT A LAWYER understanding, too. It's common for people to be caught red-handed (by 15 witnesses, on tape, and handcuffed by police at the scene of the crime) and still plead not guilty. They generally plead that there were extenuating circumstances rather than claiming they didn't do it, period. "Not guilty" is a misnomer--it can mean "not culpable" or "not responsible for your actions" as well as "innocent."