If you're standing in line at the grocery store looking at the chocolate bars next to the register you can either grab one or not. These are your choices. If you just ate a huge pizza before you probably don't feel like eating chocolate too but if you're hungry you can't resist it. In both cases you're making a choice but in both cases the choice is influenced by outside factors that you did not choose.

What does it mean when you say that you could have done otherwise? If you're super hungry and can't resist buying the chocolate bar then that's the only thing you could have done. If you go back in time to the same situation and nothing else changes then you're still hungry as fuck and are going to grab the chocolate bar. Something in your circumstances has to change in order for you to act differently but it's still outside force affecting your behaviour.