I've observed the same thing, and was wondering, myself. At first glance, I thought it might be a matter of miai -- distance between attacker and target. Closer target enables you to attack from the front hand rather than a step-in-and-swing.... But at point-of-contact, nage usually is in ai-hanmi, to be able to be on the outside of the attack (rather than the vulnerable inside). Yet, I have seen techniques where nage enters on the inside of the tsuki or shomen attack (for, say, a sumi-otoshi throw).
Perhaps the hanmi is nage's choice, depending on their preferred response-style to the attack?