It could just be that she didn't bother paying her last several bills, got a bill for $400, paid it, and then her account wasn't credited in time for the next bill -- meaning it looked like $500 more dollars. They're slow to cut off service for non-payment, especially if it's government-run ("calling the city" makes me think it's not a private company, though I don't know that for a fact since it varies across Canada). But they will do it eventually, typically after sending several notices to demand payment.
So it could be on her mind because she got a first notice, paid it, and now got a second notice that doesn't show that payment. If that makes sense.
And there's no way she'd be able to do the math, subtract the money she knows she's already paid, and then pay the balance.