after the first year, which is all the study looked at.
For one, BYU sends a disproportionate number of students on to other schools for advanced degrees - BYU is one of the top schools in the country in this regard.
The other main factor is that BYU has a lot of female graduates who have kids during college and choose to be stay-at-home moms immediately after they graduate.
Look at average salaries 5 or 10 years after graduation among graduates who are actually in the workforce. There is no comparison.