Courage is better defined as the overall relationship with courage you've had in your life. The stolid and confident person is more likely to have overcame fear multiple times, day in and day out, which has led to his well-earned confidence. in his ability to respond to life.
I do agree that that might not necessarily be true. It's possible the stolid and confident person simply had better outcomes in life that have set him/her on a winning trajectory.
I do believe though that the shaky and nervous can become stolid and confident through repeated acts of courage, resulting in their own nervous system and brain plasticity to catch up and make them stolid and confident one day.