As a teacher who spends most of your time interacting with students, planning lessons, meeting with parents, etc., it can be easy to forget that, with the exception of public schools and not-for-profit organizations, schools are businesses. And like any business, they’d like to keep costs down where they can and this includes teachers’ salaries.
Of course, they have to weigh this against the market rate of a teacher with certain qualifications and experience. This means that there is some grey area around salaries and some room for negotiation. The thought of this can be a little scary for a lot of people, particularly if for people that have never negotiated before.