Not an ignorant question at all! This is actually a subtle problem in switching regulator design: high duty-cycle operation and efficiency. Basically, if you in

Not an ignorant question at all! This is actually a subtle problem in switching regulator design: high duty-cycle operation and efficiency. Basically, if you input a voltage very close to the output voltage, the switch part of a switching regulator must be always or near always closed. And, any inefficiencies such as voltage drops in the power path are visible in the output. Being Apple, I'm sure it has 100% duty cycle operation. However, it's likely not optimized for hat (uncommon case). [edit] as the other comment points out, my comments forgot about the PFC circuit. But on switchers without this, near output voltage input is possible.