Data usage and Wi-Fi

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My g/f and I use a wi-fi connection while were at work. Her current usage of data for this month is over 600 mbs while mine is 70 mbs. For the past 5 months my data usage is always below 80 mbs while her's is always above 400 mb's, the majority of her data usage is at work.  My g/f says she seldom uses her iPhone at work, when she does its only to text and make phone calls which our phone bill indicates but I use my phone to access the internet allot, text and make calls during the course of the day so why is her usage so much higher?