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.

Well a 'bow' is a weapon you use. 'Bow and arrow'. So maybe it has a metaphoric meaning like she's strong and well-made. Apart from that, 'true-made bow' has no literal meaning.