Looks like it comes from IEEE C57.13 permissible error. Here, the graph shows 0.6% accuracy region for currents less than 100% for 0.3Bxx CT with 4.0 rating factor. "The limit of permissible error in a current transformer accuracy class has one value at 100% rated current and allows twice that amount of error at 10% rated current". Accuracy class is stated at 100% rated current. Twice the error is allowed at 10% rated current.