In fact, this question on the website makes no sense at all. The montana as well as the SF readings are within the noise of the sensor. ugh. sigh. FUD.

"Q: Wait a moment... why are Montana readings quite a bit higher than in California? A: SF Bay Area is a coastal region, while the other location is up in the mountains - which means that there is less atmosphere to stop cosmic rays. The difference in measurements is quite substantial, around 40%. If that freaks you out, consider that a commercial flight easily exposes you to about 300 µR/h!"