The plea you enter in a criminal court has nothing to do with whether or not you actually, in reality, committed the crime, that is went through the actions and 'did it'.
Your plea has to do with whether or not your defense attorney or defense team thinks they can win, that is, get you off all charges, or get you off on lesser charges.
A Not Guilty plea doesn't mean at all that the person didn't do it, know they did it, and a number of other people (witnesses, videos) know they did it too.
I have to disagree with entering a not guilty on whether or not the atty thinks they can win. You would enter a not guilty generally for any charge unless you plan on either admitting guilt or not challenging (which would be a nolle prose - my spelling may be off on that) the charges. If you enter a not guilty, you can always later change your plea ... not being in the crim law world, I would think it's just a standard thing to do NG and if anything, it might give you a tiny, fractional leverage position depending on the evidence of the crime.