I should have been more clear. 20/20 refers to vision and is used in the traditional form of the expression and is supposed to mean "very good". 50/50 refers to

I should have been more clear. 20/20 refers to vision and is used in the traditional form of the expression and is supposed to mean "very good". 50/50 refers to percentages, meaning 50% chance of being correct and 50% chance of being wrong, which would be very bad.