What matters is the volume of water placed onto plants, not the duration of watering.
Given constant water pressure the two will be correlated. However constant pressure is an assumption. There are many potential causes of reduced pressure, some of which can dramatically reduce available pressure:

• Filling the pool while irrigating.
• A well that is pushed beyond its replenishment rate.
• Running an open loop geothermal system (from same well).
• Running a second uncoordinated irrigation system.
• Neighbors on the same street.
• Town water pressure is running low.

in some cases potentially pressure may drop too low to even activate a sprinkler head resulting in zero water being delivered. 

To avoid the too low pressure problem it may be that someone temporarily shuts off the irrigation supply at the main (not via Rain Machine), e.g. to make it faster to fill the pool -- e.g. someone without access to Rain Machine who has come to open the pool. Or maybe workers painting within range of the sprinklers temporarily shut off the main irrigation value.

In all of these cases if Rain Machine was to measure gallons delivered (if there is a flow meter installed) rather than time a valve is on, then programs can be re-run or extended until sufficient water has been delivered. If after some extended period insufficient water can be delivered to a zone an alert could be generated.

Again, I don't care how long a valve is activated for, I care how much water is delivered.

- Mark