Politeness can be passive if you agree to inconvenient terms out of fear of offending. Assertiveness is "honest politeness." You respect others while clearly stating your boundaries and needs without sacrificing your interests for someone else's comfort.
If confidence turns into pressure, ignoring others' feelings, or hostility, it becomes aggression. True assertiveness always involves a balance between defending your rights and respecting the rights of others.
Yes, cultural norms and upbringing significantly influence communication styles. In some environments, directness may be mistaken for rudeness. The Rathus test helps evaluate your personal effectiveness in communication regardless of ingrained patterns.