The lesser charges were included and the jury could have convicted her of lesser crime than first degree murder if they wanted to. As it is the jury apparently didn't think that FCA was guilty of anything except lying to police, even child abuse.
I think the jurors have said that they thought the possibility of an accidental drowning or another way and the prosecution have failed to make their case.
It's not so much as not guilty as the prosecution not making their case.
Casey Anthony is a case that 99 times out of 100 is pleaded before it ever makes it to trial typically.
I've never understood why the grandmother waited a month to call police. If she was truly as alarmed as she said she was on the 911 call, why the long delay?