Unlikely; my tap water is well water. I do clean the humidifier with vinegar every week to keep the mineral buildup down. Then again, I do that with the shower head and tea kettle too (though a bit less frequently).
The big thing, as I've found it, is keeping the output down to a visible minimum. A big plume of mist that visibly lands anywhere isn't adding more water to the air - it's putting water droplets on the surface much like a spray bottle. It looks good, but isn't doing the job you're asking it to.
Then it is probably a difference in ultrasonic humidifiers. I can tell you that mine barely builds up much slag at all. I switched to warm-air after I grew tired of the white-dust issue.
The white-dust issue is further confirmed as I use the warm-air (boiling-type) humidifier. A LOT of minerals are found every day from my boiled-away tap-water... minerals that clearly used to be sprayed into the air and dusting my furniture.
Warm-air humidifiers are less efficient: My room is ~25% without any devices running. An ultrasonic would bring it up to 35%+, but Warm-air humidifier only brings it up to 32% or so.
Another efficiency test: both units were 1-quart (EDIT: Not a gallon) units. The warm-air humidifier probably goes through the tank in ~12 hours, while the ultrasonic would go through in ~8.