For instantaneous measurements, the width takes the region under the mouse cursor and finds the nearest transitions on either side. That distance is the width result. The period result takes that width and adds to it the width of the next region to the right of the mouse cursor.
For the persistent measurement, width and period become the time between the start of the measurement and the end of the measurement and don't have much value. For persistent measurements, please take a look at the minimum, maximum, and average measurements, which will analyze each cycle within the measurement instead.