I would think so, too, except the same error is consistent as I move from town to town. The iPhone changes the location as I move, but the temperature is always running 3-5 degrees F higher.
For instance, Yahoo Weather is reporting the temperature in the town where I live is 1 degree F warmer than my work location, so it is obviously running off of a different sensor. But they are both consistently just a few degrees high. This is true wherever I go in the area.
If I hadn't been noticing this consistently over the last couple of months, or so, I would think it was nothing. But it has been too consistent for too long a time to dismiss it as simple error.
The weather site goes with the temp of the National Weather Service monitoring station closest to your area.
Some global warming skeptics, a few years back, discovered that the NWS monitoring stations were badly located, sitting near warm buildings, air conditioner exhausts, etc. This would skew temperature readings upwards. Report here
It looks like you've noticed the same. The "official" temp readings are skewed to show "warming".