Your comment is noted and taken into consideration, perhaps they want to measure the positive sequence component of resistance. Are you familiar with symmetrical component vector notation mathematics. The system is a method of describing any complex three phase circuit by 3 complex component vector values. i.e. a positive, and negative and a zero sequence component. It is similar to expressing a vector quantity like (R + Jx) except that where Jx is used, it is replaced with A0, A1 and A2 and A is 0. 120 and 240 degrees instead of R= 90 degrees. This wattmeter method would provide the positive sequence values. This is just a comment, I am trying to justify a reason for their method.