This is a routine occurrence in aviation. What happens is that one particular operator (airline) discovers an issue (in this case an electrical grounding fault)

This is a routine occurrence in aviation. What happens is that one particular operator (airline) discovers an issue (in this case an electrical grounding fault) and decides to conduct a local survey of their own fleet. If they discover more than one aircraft is affected by the same fault then they will usually choose to notify the engineering authorities who will then send out an airworthiness directive, or a service bulletin, or similar instrument to all affected parties. Because only certain aircraft were affected with this issue it's probable that it has to do with a specific variant configuration of avionics in use by only a subset of operators.