He heard because he wanted to hear. His ears were acutely attuned to hearing. Everybody heard what they wanted to hear. Yisro heard the truth, because this is what he wanted to hear. Horav Yosef Leib Nendik, zl, remarks that this is, in fact, human nature.
People hear what they want to hear.
Imagine, a group of people hear that an individual became very wealthy. His friends will obviously rejoice in his good fortune. Businessmen will be happy to hear this, because now they will seek his business. Fundraisers will be happy to hear they have someone new to ask for money. Hearing is an art.