What fear caused Penelope to hold back her belief it was Odysseus for so long?

Penelope must contend with her fear that she is being duped by someone who is not actually her husband. After all, Odysseus has been gone for twenty years, and he would likely look very different after such a length of time; he would have gone from being a young man to a middle-aged one. When he arrives at their home, claiming to be himself, he is able to string his bow and shoot an arrow through a row of axe handles, but still Penelope doubts his identity. She issues an additional test; she tells a servant to remove the bed she shared with Odysseus and place it in another room so that he can sleep in it. The real Odysseus would know that this is impossible because he had carved their bed into the trunk of a tree that grew up through their house. When Odysseus becomes angry, insisting that such a thing would not be possible, she knows that it is, indeed, him, because no one else knows the details concerning the bed.