Spend enough time around the criminal justice system and you'll soon learn that there's no such thing as "an open and shut" case.
FWIW, from what I hear, the attorney prosecuting the case didn’t want a conviction.
Spend enough time ... there is no longer a feel for right will prevail. I have first have witnessed personal loss from a screwed up family court. Grandparents with clean history lose grandchildren because of social worker lies.
I've read up on the specifics of the case. IHMO, it appears that the jury completely disregarded a jury instruction from the Judge.
A big piece of the defense rested on Tiller claiming that he relied on advice from his legal counsel before engaging in a business relationship with the consulting Doctor. Of course, even though the jury was suppose to disregard it, they still heard it from Tiller's mouth when he took the stand in his own defense.
I bet if you put the jurors under a polygraph, this is the reason they acquitted. - Again, there are no open and shut cases. Unfortunately, many times there should be.