Again, this is a gross oversimplification, but it gets the point across.
Let's say a stereotypical single income LDS family tithes $50k gross annual income. Net untaxed income = $45k
Compare that to a stereotypical dual income non-LDS family where both parents make $50k. Untaxed, untithed income = $100k
LDS family nets 45% of non-LDS income.
Obviously there are broad variations from both stereotypes and many factors affect net household income not to mention differences in spending habits.
Still you're going to see LDS families tend to have lower net household incomes than similar comparable non-LDS households.