Why? easy - two of the 155 operators on the SNS had a number of overtorques in light wind conditions (fenistron et al) and so a payload penalty was imposed when winds were less than 10 kts. It was found that this payload reduction was not enough in less than 5 kts, and so a further reduction was imposed by the operators. The amount of reduction was set by the individual operators, but was up to 300kg.
By the same token both the 76A+ and B models had wind limitation payload penalties, to varying degrees.