It is as far as I'm aware a reading of the minimum steps per minute, and the maximum steps per minute over the course of the run.I assume the SPM reading you have there is for two steps not one.
Otherwise something is rather odd.
There is also a graph which compares Heart Rate, Step Rate in conjunction with the Heart Rate.
So if I'm running uphill you can see how the stride Rate shortens, so more steps per minute, with a corresponding higher heart rate. It also overlays the altitude. Quite impressed to be honest.
Although the main reason I got the watch was for the GPS to measure the total distance run.