I don't know how accurate fingerprint readers can be, but I know that the ones in our office require you to put down your finger perfectly square on the reader and that you remain absolutely still for the time it takes for the reader to work.
If these are the same, I don't see any way that they could ever get an accurate reading if you just twitch your finger every time they place it on the reader. Even holding your fingers down in place, they would have a tough time to keep you from voiding any potential read on the fingerprint. Smashing your finger on the reader causes the fingerprint to squash and also become unreadable ... take it from me, I've done it by mistake and voided the read.
The iPhone finger reader is very good. Angle doesn’t matter, it’s very fast, and at worst takes a couple repeat touches to engage. It doesn’t read the print per se, it reads other structures deeper in the finger. People only understand “fingerprints” in the context, so that’s the term used.