Here is how The Associated Press Stylebook (2002) lays out the rule you refer to in your question:
apostrophe (') Follow these guidelines:
SINGULAR COMMON NOUNS ENDING IN S: Add 's unless the next word begins with s: the hostess's invitation, the hostess' seat; the witness's answer, the witness' story.
This rule governs two of the examples you ask about:
- the bus' seats (referring to just one bus)
- the mattress' springs (referring to just one mattress)
But your other two examples involve possessives of plural (not singular) nouns ending in s, so the cited rule doesn't apply to them. Instead, the relevant AP rule is this one (from the same general entry on apostrophe use):
PLURAL NOUNS ENDING IN S: Add only an apostrophe: the churches' needs, the girls' toys, the horses' food, the ships' wake, states' rights, the VIPs' entrance.
This rule governs the other two examples you ask about:
- the buses' seats (referring to more than one bus)
- the mattresses' springs (referring to more than one mattress)
So the answer to your question is that the four examples you ask about are indeed handled correctly under AP style as given, but they are correct in accordance with two different specific guidelines: one governing possessives of singular common nouns that precede a word starting with s; and one governing possessives of plural common nouns, regardless of the word that follows them.