The technique of tuning youself slightly sharp of other people you're playing with is a well-established dastardly trick (I've done it myself). It definitely makes your contribution to the overall sound a little brighter and more upfront. I feel Joseph Spense was doing this to himself: certainly those with keener ears than me suggest that he was entirely consistent with his tuning, and the basis of it was to tune the top three(melody) strings in with each other, but fractionally sharp of the bass strings. Maybe he just had a guitar that was a little unresponsive in the treble when he was defining his style. This tuning makes the tune sing out a bit more, at the expense of sounding a bit jangly. And as he confined his playing to one key and one tuning, he could control things OK.It would have sounded a lot worse if he'd tried playing in another key.
Next burning question: why did he invent that style of singing?