Why do people keep mixing up trans-gender and transual?

It's quite hard to understand anything about either I suppose, unless you are trans. I can feel sympathy for a trans person but I can never really empathise as the thought is so alien to me. I understand what you are saying is the difference between the two -gender is identity rather than physical- but they are generally used interchangably, even by many trans people so it's all a bit confused. Wouldn't s by definition also be ?