Way back when, children did not have finger scans because their fingers would outgrow the scan data from the last visit, resulting in added delays at the turnstiles. The quickest way to resolve problems back then was to reset the scanner to treat the ticket as new first time usage.
Actually I think it would expedite things today if during busy times the finger scanners automatically gave a green light after, say, three quarters of a second,, unless the the blue light criteria was satisfied more quickly. The delay would be so people thought that the scanner was still active.