What Capital One is seeing are applicants who are opening accounts (too many new revolving accounts), but not using them ongoing (no revolving accounts have a balance). This indicates churning, and they aren't interested in awarding new account bonuses for card that will not be used after the bonus is earned. It doesn't matter to them if their algorithm is correct in your case, each of us is nothing other than an instance of a type.
I think you just figured it out. Cap One's problem with me isn't that I pay off balances, it's that I have more credit cards with zero usage than non-zero usage. Hmmm... Next time I'll get some charges on all cards for a few months before applying for a Cap One card.