The NFL stats definition of a dropped pass is pretty strict, it has to be a clear drop by the wide receiver as opposed to a bad pass by the quarterback or one that goes off a receiver's fingertips. Looking at the percentage of dropped passes per attempt (percentage) is a more effective way of evaluating incompletions that resulted from dropped passes.
More about Dropped Passes by Quarterback
Quarterback Passes Dropped Percentage measures the percentage of a quarterback's pass attempts that were dropped by the targeted receiver and lead to an incompleted pass.
Passes Dropped % = Dropped Passes/Pass Attempts