I believe your list is a bit off on this. Taking into consideration the number of trained troops along with computing power for OFFENSIVE cyber operations capability. The list is probably closer to being: Israel, China, USA, Russia. South Korea could be in the top 5-7 within a few years with the help of China.
The USA isn't number one as they were late getting into the OCO arena, and are just now starting to get a large number of people trained. However, the USA by far has the best defensive cyber operations capability, computing power, as well as some other aspects of the cyber war game. DCO was given a higher priority than OCO until a few years ago, when OCO began to become a priority.
USA also has a disadvantage of military downsizing, dwindling funds, and there are some war legalities in allowing civilians to do targeted offensive attacks. So these types of operations must come from military personnel.