Well, if each axis of happiness / anxiety / stress is equally important, then the happiness gets canceled out. And you'd wind up with a score indicating as much on the POMS.

This seems sensible to me. If the happiness wasn't being canceled out by the other two, would you really be feeling 'kind of stressed'? Wouldn't you be feeling a kind of relief or smugness - 'ha, beat the deadline again!' - or feeling of accomplishment - 'go me!' - or something positive like that?