Here is how ACT defines these groups:
Essentially, a student who has an expressed interest in STEM [has chosen] a major or occupation (out of the 294 listed in the Standard Profile Section of the ACT) that corresponds with STEM fields. A measured interest utilizes the ACT interest inventory, an inventory delivered with the ACT that determines inherent interest in different occupations and majors.
"Expressed and measured" means that the student's interest shows up both directly in the standard profile and indirectly in the interest inventory.
The complete study is available from ACT's web site. By the way, Ohio's students are better prepared for STEM study than the nation as a whole.