Why not Pa-Pb in the answer.

[Sharmaa]

Here is the problem, am bit confused why they did not use Pa-Pb in the solution. I know in abc seq if the voltage coil does not go with seq letter then that wattmeter reading should be negative. Per than for Wa is between a and b so should be positive but Wb is between c and b instead of b and c , so should be negative. Per this rule it should be Wa-Wb not Wb-Wa. Correct me if am wrong or this solution is wrong.
Thanks in advance