That page in your link is headlined with “School of Professional and Extended Studies”. It is not necessarily intended for reference by incoming freshmen. It appears to be a price page for a specific program for visiting students.
The page most aspiring freshmen would likely see, accessible from the main undergraduates admissions page is this one: https://www.american.edu/finance/studentaccounts/tuition-and-fees-information.cfm
It is not intuitively clear on that page that the prices listed are per semester. There are three groupings on the far right of the page that list “Payment Due Dates” for Fall, Spring, and Summer, but it does explicitly state whether the prices listed are per semester or not.
Instead, the page is headlined by “Tuition Table 2019-2020 Academic Year” which could easily be interpreted (or misinterpreted) to mean the prices listed are for the entire year, not per semester.