If your designer is stuck in iOS pixel-perfect land, use this number to convince them they need to learn to use Asset Studio and live the RelativeLayout way.
I would have said "models", not "devices". But the point is clear -- there are a lot of different configurations and hardware, all described as Android devices.
I regularly hear from people who say one of my Android apps won't work on their device. At first I would try to adjust my software, but as the years have gone by, I realized there are too many fringe cases that don't have anything remotely resembling a standard set of features -- even if they're formally running some Android version and have some "standard" set of hardware features.