Here is my opinion on that. Just an opinion. Anyone that has ever been drunk can vouch for this. I am not talking about a blackout, I mean just drunk, somewhere between tipsy and passed out. If I drink too much, the next day my memory of the night before is very hazy. I was not in a blackout state (that is much different)but there are things I have trouble remembering. For instance, if I were to have say, 4 drinks, I would be drunk. The next day when going over what I recall about my evening, some stuff I'm just not going to remember. I may clearly recall putting my pajamas on, but dont recall taking out my contacts. I know I did, because they are in the case, but I sure don't remember doing it. Or lets say you are at a bar and get drunk. Maybe you remember getting in the car (with a designated driver of course) and you remember going to your front door, but the ride in between is pretty hazy.
This is what I think is going on with DB and why the timeline changed. She knows she put Lisa to bed at 6:30 and she remembers it was 10:30 when she went to bed or thereabouts. But the time closer to when she went to bed is more fuzzy, because she was drunk. She THINKS she checked on Lisa, Im assuming because it was something she normally did as routine, but she can not clearly remember walking into that bedroom and checking on the baby. So she doesnt want to say yes she did see her, because truthfully she is not sure, because she doesn't REMEMBER.
THIS is just my OPINION.