Timing with multiple Pulse R's at the same rate.

Each Pulse R has its own processor (brain), and each processor has its own internal timing, known as a clock rate.  The clock rate is what the Pulse R uses to determine the timing and when to fire the output.  While all the processors have the same face value clock rate, the actual clock rate varies from processor to processor by a few microseconds due to variances within the manufacturing process.  These differences start to add up over time, resulting in the timing of one Pulse R being slightly faster or slower than the other. You would see this same effect if you started two metronomes at the exact same time with the same beats per minute or started two of the exact same car blinkers at the exact same time. They would eventually come out of sync with each other.

Once the signal/power to the Pulse R has been turned off and back on again, they will start back up in sync and slowly work their way back out of sync.