Much of this area sits on heavy clay that swells when it's wet and shrinks when it's dry, and that movement is what leans your fence years later. Posts get set for the soil that's actually on your property, not to a generic spec, and on wet ground we would rather wait a day than set posts in mud and watch the line lean two summers on.
Your property gets measured and the number comes to you in writing before anything starts.
A post that has worked loose in wet ground near the bay is usually a repair, not a reason to start over.