Instead of taking a step-and-read approach, the RPLIDAR drives the rotating scanner with a DC motor, continuously taking readings and making note of the scanner's angle with each one. Because of this approach, a single revolution is not guaranteed to give a reading for each possible angle, but over several rotations a full scan can be assembled.
The animation below shows a different mechanical approach (using a rotating mirror instead of a rotating sensor) and only covers 180 degrees, but the concept is identical. It shows the general theory of operation: readings taken at different angles, combined to provide a map of distances from the sensor. Using this data a system can get a sense of the space around it.