This seems to me to be a sign that they're not really mature minds because it reminds me of a depressing commonplace: a small, angry child screaming that his or her parents are lying because they made promises fate couldn't allow them to keep. A little girl doesn't really have the ability to notice what's going on behind the scenes and doesn't realize that Mommy and Daddy aren't really in control of all that much and thus can't see the thing that makes the promise untenable.
Said little child is, however, perfectly capable of turning around and insisting that things aren't her fault when she's forced to break a promise because she can see the obstacles when she's the one they're in front of. This is why Mike can witlessly assume "Hmmm. Since Lizzie came along, Mommy and Daddy have always been on my case so Lizzie must have deliberately come along just to see me suffer" while also wondering why it is that his mother assumes that every single thing HE does is because he hates her and wants her to suffer. Most people grow out of that because most people have something Mike and Elly lack: brains.