Explanation: Despite practically wielding near-absolute autocratic power, the Romans were very firm on the fact that the Emperor was not a king. In the Roman conception, a king was a superior, while an Emperor was merely 'first amongst equals' who had been given 'by consensus of the Senate and People' absolute power with no checks or balances that the Emperor didn't feel like honoring any given day.
Absurd as it may seem, and it is a bit absurd, this did have the effect of limiting the Emperor's power - all power rests on the perception of legitimacy, and in order to appear legitimate, the Emperor had to give some amount of respect to the idea of being 'just the First Citizen'. At least until the later 'Barracks Emperors' of the Third Century came about, and all pretense went out the window - along with any ideas of government legitimacy beyond "I have a sword, and you don't".