The gas bars, does this happen to anyone else?

[Ilovemyyaris]

Does anyone else ever witness a gas bar disappear while you don't have the car on?

Okay so tonight I was sitting in my Yaris cleaning the inside. I did NOT have the engine running but I did have the radio and lights on. Well I had a full tank of gas "all 8 bars" and when I was done cleaning/vacuuming I just sat in the front seat let it back and relaxed a little. I sat there for about 5 minutes and I was about to leave when I noticed one of the bars was missing.

It's done this once before, then the bar came back when I drove a little before actually disappearing. The last time it happened the same situation, I was sitting inside without the engine running and a bar disappeared.

Is this normal? This is the first car I've actually ever owned, so it might be normal for all cars idk. It's a 2011 model 5 door hatchback. It's a minor problem, I just want to know is it normal or just happens with mine?