Why did a police officer charge me for not having an insurance card, after I gave him it?

Tonight I was pulled over by a police officer in a residential neighborhood, and since it was the first time this had happened to me I was a little nervous, not mention unsure of how to proceed. When he asked for my license, registration, and insurance, I handed him my license and registration, but being genuinely confused I said that my mother had my insurance card (mistaking it for my health insurance card which my mother had taken the day before to pay my doctor). He asked me why a doctor would need my auto insurance, and then demanded I "try again". Truly baffled I happened to look down and see my auto insurance card in the plastic envelope which I had my registration in. Finally understanding, i handed to the officer asking if that was correct, and he took the up to date insurance card out and left with the three without a word. He came back a few minutes later and handed me a summons for not having my insurance documents. Just happy to have it done with, I thanked him and he left, but then curious as to what int he world just happened I called my parents. My mom asked if the insurance was expired, I replied no. She too was confused as to why the officer gave me this for not having my card, even though I did, after some initial confusion, but before he even left the side of my car the first time. Any insight as to why I was charged with this?