How would you feel about this?

Girlfriend works as a nanny. The dad is a difficult man.
Today he said his calculations of her working hours are different to his by about 10 minutes or so. She said to him quite rightly that it's always going to differ in a small way between his and hers. He agreed and said he would always go by what she said the hours were but she's been left feeling insulted and upset. Would you say she is over reacting? I can't help but feel the same as her