How could this bubble level be wrong?

When we had the RV we would level it at the camp site. Rick relied on the bubble level to tell him when it was level, back to front, and left to right.
It was always spot on back to front, but invariably it would be just a little off, left to right. I’d walk in and say, “It’s not level. It’s tilting that way.”
We’d argue and I said, “I don’t know how a bubble level could be wrong, but it is!”
I dropped something round on the linoleum floor, and sure enough it would roll that way.
So then he would adjust it until I called it good even though the bubble level was just shy of center.

How could something as simple and basic as a bubble level be off?