This inability to remember the past marches hand in hand with the assumption that everyone else has a memory like a sieve. Lynn's suppressed anger at having things she's said in the past brought up is combined with confusion because, well, she's upset that people seem to remember things. This sort of not really thinking and not really remembering is thus another factor that makes the notes an exercise in confusion. Not only do we have to deal with things that only make sense to her, we have to contend with her inability to remember why she did things.
One of the things that she seems to have failed to remember is (as forworse implies) that she even had a character named Janice to play with in the first place. By the time she got done hammering the kids over the head with the fact that they should be grateful that John and Elly feed, clothe and house them and us over the head with the fact that John is an insensitive brute, the concept "tomboyish foil" seems to have vanished from her brain. This, along with an antipathy to tomboys I'll discuss later, is why the next time a girl named Janice appeared, she was more of a real girl.