about Salt Lake County. And just because you have a less reputable source, does not make it true. When the vast majority of reliable sources point to something, it is more likely to be the case.

As for your first source, if you look where it gets its data, it is by membership records of the church. The church has that many membership records in the state of Utah, so it counts that many LDS people being in the state.

This is much more flawed than any of the other sources for 2 reasons. First, many church records in Utah are of people that don't actually live in Utah. That is usually just their last known address or the address of their parents.

Second, there are many people that may have technically been baptized and have a church membership record, but they have then jointed another faith, or have no faith at all. They just have not gone through the formality of removing their record (most people don't even know that is a thing). In other words, someone in the world population review report that may have joined another faith, or become an atheist, is still being counted as LDS. The other "reliable" sources are a much better measuring stick of true numbers.