I can kind of see it as a sort of baby-talk nn for Elizabeth, and maybe even for Katherine. Katherine only because when I was very little, just learning to talk, I called kitty-cats ditty-dats. One of my uncles liked this so much he started calling kittens dittens.
"It's one thing to be open-minded and quite another to be so open-minded your brains fall out."--Dear Abby
"Let other people push you around, and you deserve whatever bad things happen after that."--Lauren Bacall