(Strip Number 5097, Original Publication Date, 8 March 1989)
Panel 1: A few seconds later, we see Elizabeth looking at the wreckage resulting from her inattention and thought-bubbling about how having broken the milk carton Elly sent her to get, she's going to get it.
Panel 2: She remembers that Elly has a volatile temper and no real tolerance for screw-ups by telling herself that Mommy is going to kill her. (She is, of course, exaggerating. Elly's anguished ranting about how much it hurts that she can't be trusted to do ANYTHING right because she's BAD and HAAAAATES her poor mother will only make Liz envy the dead.)
Panel 3: She cries because she has to go home and tell the truth and listen to years of angry, self-righteous screeching about evil children who lie about there being anything like accidents in this world.
Panel 4: That is, of course, unless she comes up with something better than the truth.
Summary: I should think that the reason that this makes sense to Elizabeth is not just because she wants to avoid getting spanked for not being trustworthy. I think that she confuses Elly's tendency to fold up and die when it actually matters with Mommy having the IQ of a tub of Spackle.