One of the many meanings of 'true' includes the idea of 'straight'. If a bow is made true, it is made correctly, so that the arrows it fires will reach their destination accurately. A bow which isn't true is distorted or malformed, so that the arrows won't fly in a straight line.
While Odysseus wandered, Penelope remained 'true' in all its meanings: faithful, fixed, reliable and constant. It's a beautiful metaphor.