Why do different calculators give different answers to simple math problems like 6 divided by 2 times 1 plus 2

Im a teacher and my students showed me something weird. If you type:

into different calculators, you get different answers! My Casio says 1 but my phone calculator says 9.

I thought math was supposed to be universal. Why does this happen? Is this a calculator bug or is there actually ambiguity in how we write math expressions?

Which one is correct and why do engineers vs mathematicians argue about this?