Part of this discrepancy could have to do with the younger demographics featuring more working individuals, as the festivities measured happened between 10 a.m. and 6 p.m. ET.
Per Variety, the smallest audience viewership for the Trump inauguration was the 18-34 demographic, which only saw 2.8 million viewers tune in.
19.2 million viewers tuned in.
It’s also a measure of how people may have voted by age, a full breakdown of which can be found here.