Same thing with the burners on a gas unit, though. It’s not the bottom of the water heater that needs to be raised 18", it’s the “source of ignition” that needs to be raised 18". However, since all units are different, it’s easier for contractors to simply install 18" platforms everywhere and site the water heater on them. That way they don’t have to get into measuring the distance from the floor to the “source of ignition.”
That is, indeed, another “source of ignition,” so whichever is lowest is what needs to be raised 18".
That’s also why everyone simply puts the water heater on an 18" platform. Make it easy and consistent and be done with it.
And the only reason why the “source of ignition” in dryers is not required to be raised 18" is because they are considered “attended appliances.” I hate to tell the code people, but when I use the dryer, I turn it on and leave. The only time I’m attending it is before I turn it on (loading clothes) and after it has turned itself off (clothes are finished drying).