Here's my line of thinking. Not saying that I'm right, I'm just showing my line of thought.
Played yesterday and had an uphill lie. Usually I hit these with a big draw, often times they start out to the right and draw quite a bit back to the target. But this time I hit it straight at the target and it went dead straight. Looked at the divot, divot pointed out to the left.
Then I thought about some things. First off, the normal flight on those uphill shots seem for me seem to be the same for others. This type of flight where the ball starts right and then you have a draw to me seems pretty similar to somebody with a lot of shaft lean and/or coming down steep at impact as the horizontal plane naturally moves to the right and swinging left will get the ball to straighten.
And when I was thinking about it further, we are told to try and get our shoulders parallel to the ground on uphill and downhill lies. With the uphill lie, that means we need to tilt the shoulders which promotes more of a swing that 'hits up' on the ball. I would think that the reason being is that if you were not to adjust your shoulders, you would have too steep of an angle of attack.
3JACK