Few percent of transactions for a payment processor means a few billion transactions. Visa on its own processes ~200B transactions a year. That's not a great threshold.
It's a trade-off though, right? Presumably it's better for the company to lose out on a small percentage due to false positives than to allow them through and subsequently incur a larger percentage of false negatives. They can aim for obtaining those transactions without incurring extra fraud but it has to be worth the cost and risk of the changes to the system.