not necessarily so; is not always true
Says that something is not always true, despite appearances or common assumptions.
This pattern expresses that a general statement or expectation does not always hold. It is often used to counter a stereotype or a common belief, meaning 'not necessarily', 'not always'. For example, if someone thinks expensive things are good, you can say 高いものがいいとは限らない (Expensive things are not necessarily good). Note that it attaches to the end of a clause, and the clause is in plain form. For nouns and な-adjectives, you often see the だ omitted, giving a more direct connection.