If somebody or something is worthy of trust, then you may / will be able to trust them. The distinction suggested above for senator Whiplash is meaningless because trustworthy does not mean it/he,she,they is/are unconditionally 100% trustworthy ad infinitum for everything, unless explicitly stated. Neither is trustable elsewhere defined as only being used when someone/something has a defined limit of trustworthiness. A logical reason for the new word trustable to evolve could be that people do not use worthy in as widely as they would have say 100 years ago. Today you would mostly describe a person or product as trusted, reserving worthy for distinctions carrying honour (medal, knighthood, prize). When worthy is used as it were 100 years ago outside of that context it appears stuffy or pretentious today, (the worthy dog vs a trusted companion, worthy choice vs good/excellent choice).