Can someone explain this to me?

Ok so I started working at this job where another girl was a supervisor and im going into a supervisor role and as I was sitting there keying some financial information in with assistance from another supervisor she was saying just please don’t be a jerk and I turned and looked her in the eyes and said that’s not in my personality and it’s not I’m very friendly but one thing I don’t understand is telling me not to be a jerk then talking about me behind my back when she thought I couldn’t hear her which I was only a few aisles over
Can someone explain this to me?
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