The Hechinger Report has noted these sources of variability in mathematics classes:

Anxious teachers may spend less time on math: “Math specialists say [math anxiety] is a pervasive issue in elementary classrooms, where educators are typically expected to teach every subject, and it often leads to teachers spending less classroom time on math content.” https://hechingerreport.org/teachers-conquering-their-math-anxiety/

DIY instructional materials: “teacher-made materials may sacrifice the thoughtful sequencing of topics planned by curriculum designers.” https://hechingerreport.org/proof-points-many-high-school-math-teachers-cobble-together-their-own-instructional-materials-from-the-internet-and-elsewhere-a-survey-finds/

I’ve discussed other reasons for variability here: https://mathvoices.ams.org/teachingandlearning/is-data-science-the-new-discrete-mathematics/.