That was a joke. Let me tell you the reason why it is funny...
My boss said my math skills are average...
My boss said my math skills are average... That's just mean.
This is a pun, and also a definitional joke. The pun is centered around the word "mean". Mean is a word with multiple definitions. The two that are being used here are "unkind", and "the average obtained by summing and dividing by the number of things you summed".