I'm an ignorant layman here, but the uncertainty principal only says that if you're sure about the size/location, then you're very unsure about the momentum. It

I'm an ignorant layman here, but the uncertainty principal only says that if you're sure about the size/location, then you're very unsure about the momentum. It's possible that measuring its size does not need much certainty about its momentum, so they can get the accuracy they want.