Hmmm... I don't know if this will be of much help, but I do have an answer! =)

It's easier for me to hit numbers on the Y (vertical axis--20 & 3) than on the X axis, because it's easier to accurately control the left-right trajectory of the dart than it is the vertical, which is where I'm dealing with gravity--I can't aim straight, I must aim slightly above the target for it to "fall" into place--so I'm likely to throw slightly high or low (or too hard or softly), and therefore much more likely to miss my target, if it lies along the X axis.

As far as the oblique numbers, I find numbers near 16 or 18 easier to hit; their "slices of pie" are more in line with the way my arm moves as I throw (down and to the left; I'm right handed). Although I may miss the exact spot I'm aiming for, I'm more likely to hit something in the number, because if I throw high (early), I'm more likely to throw towards the right as well; if I throw low (late), I'm more likely to throw towards the left.

I find 15 and 17 (and upper-left numbers) more difficult to hit, because they are crosswise to to the axis of my throw.

As far as the cognitive psychology goes, I don't see how location memory is relevant, as I'm not throwing at a target I *remember* seeing; I'm looking directly at the target as I throw the dart. There's nothing to forget.

I suppose it might be relevant if I were to look at the target, close my eyes, then throw the dart. That's not my usual game-winning strategy though. =)

Good luck with your study, and feel free to ask any more questions.