My name is Bill

Yesterday a woman came over to buy a table that Geneva listed on Craigslist. She bent over to Porter's level and said, "What is your name?" He responded, "My name is Bill." I was over by the kitchen sink and not sure that I heard him correctly. Sure enough, the lady repeated, "Nice to meet you Bill. My name is Cheryl." I'm thinking, "What? Bill? Must be a mistake." When she is leaving I call Porter over to me. I stoop down and said, "Porter, what is your name?" He holds his hand to his chest and says, "My name is Bill." I said, "No it's not. Your name is Porter." He looks at me as says very surprised, "O-o-oh!" I don't know why he thought his name was Bill.