It's not at all marketing or adversiting, it's the reality. The sensor is round, you have to crop a rectangle out of it.
Look at this illustration (for the Nokia 808, it's the same for the 1020, and, the principle applies to your 830)
Excuse me sir, but there is something that I don't understand about the sensor being round.
As you can see, the sensor is not round. The sensor cage is round but the sensor is not. Also never heard about round sensors in phones. Guess I have to study this but sensors are rectangular. Can you point some that are round? And in Lumia 830?