One of the most surprising realizations for adults who begin working in schools is that what is taught is not what they were taught or what they think should be taught. Curriculum is a part of school that is affected by many factors; some are very local (individual teachers are ultimately responsible for what it taught in their classrooms), others are school-wide or district-wide, yet others are state-wide or national. In the United States, there is no nationally mandated curriculum, and concepts like “fourth grade mathematics” are variable and open to interpretation. In some states, there are textbooks that are approved for use; in those states, all Algebra 1 teachers (for example) must select one of the approved textbooks to use in their courses.
Obviously, textbooks publishers are very motivated to have their books on those lists in states with large populations. As a result, those states can exert strong influences on curriculum beyond the jurisdictions. In addition, some open educational resources are becoming more widely-adopted, thus they are heavily influencing curriculum decisions.