"I Have Perfect Credit!" (REVISITED)

I look at credit karma every day. It's what I do. It's how I roll. And today--my perfect credit score notwithstanding--I was informed that my credit score had lowered because during the month my credit card balance went from $0 to $2100.
I am taking a credit card challenge. You know... use this card for a month and we will give you a reward of blah-blah-blah. Most of the cards I have come with some bonus value additive. The card will be paid in full with one payment before the month is over. So why should my score be affected by this? It's because these stupid assessments are made as a snapshot in time... sort of like your EKG printout while your heart is doing its thing. In the moment there could be a bleep that isn't usually there the rest of the time. You just hit an unlucky draw.
I don't give too much thought to the perfect credit comment. I have valid / active cards that are filed away in a wooden index card box. They were opened for a special in a store or something similar. Discover, MC, Visa... I never cancel them. Someone once said closing accounts can negatively impact your credit. How on earth can a plastic card that's never been used and has a $5K limit cause my score any adverse action if I decide to shut it down.
<SHRUG>
That's why my life never went into the fields of finance.