Pocket calculators: Gibberish on display

Factual Questions
1

I’ve had a Sharp calculator for more than ten years now. It works fine (it’s “solar powered”), but one thing baffles me: When I get ready to use it and open its plastic cover, stiffened much like a book cover, the display line shows gibberish–numbers, symbols, and parts of numbers on the LCD display. If I’ve just let it sit for a day or more (sometimes open, sometimes closed, it doesn’t matter), why will it start to display stuff like that all by itself?