My understanding is that wage differentials should not exist in a perfect labour market for occupations, but the following exam question seems to imply that this can be the case. A brief marking scheme also implies this.
and its marking scheme explanation:
I have always thought that if occupations belonged to a perfect labour market, it would mean that due to the assumptions of perfect mobility and homogeneity of labour, wages will always adjust so that wages are always equal across industries. The marking scheme seems to imply that this does not have to be the case.
One possible caveat I suspect would make the question's situation possible would be if the firms do not operate in perfectly competitive product markets, though I am not sure about this, and neither did the marking scheme mention this.