Harwell and Gatti (2001) stated, If the test score variable possesses an interval scale, then the difference

Question:

Harwell and Gatti (2001) stated, "If the test score variable possesses an interval scale, then the difference in proficiency reflected in scores of 10 and 15 is exactly the same as the difference in proficiency reflected in scores of 15 and 20" (p. 105). What is the defining principle that allows researchers to draw this conclusion?