[D] Why is one loss positive and the other loss negative in a multi-output branch?

I was reading section 3.1.D of this paper on re-id and I had a question about this part:

image of relevant part

Specifically, both parts are minibatch losses. So why is the 1/Nb loss negative, and why would the 1/Ns loss be positive. Intuitively, the losses are summed, so it should be (-1/Nb)+(-1/Ns) = -1/Nb - 1/Ns, right?

machinelearning negative positive