Why did they bill my mom extra when she did nothing?

It's hard to explain but my mom got an extra bill on something for no reason (it wasn't cell phone bill btw). Anyways, she's so mad now and they even told her she called them after and said she would pay it which she actually never called them because she was on vacation and was too busy to use her phone. I was there with her and she even called them after the vacation and told them she never called them before that. Idk why bill companies make up lies. Idk if the economy is so bad that it's trying to scam people's money but anyways, she's starting to find out that they made a mistake but why do they even do this? Shouldn't they be more organized before they start billing the wrong people? If that was me on the phone with them instead of my mom I would either sue them or tell them to shut the hell up and go f*ck themselves ahahahaha!