As I mentioned before, the winner of a 9-ball match isn't determined by who pockets the most balls total throughout the entire match. Rather, the winner is determined by who sinks the most 9 balls (which is the last ball on the table for the vast majority of the time). It's possible to sink more 9 balls than your opponent and yet sink less total balls. (We've all been there...on both sides!) So it's not as straightforward as simply looking at the differences in absolute potting percentages.C'mon, let's use our brains! Let's say a pro misses 5% of his/her shots, and the slightly lesser player misses 8%. That's a 95% make rate vs a 92% rate.
Now tighten the pockets. The better pro now misses 7% but now also the lesser player misses more as well, 10%. The numbers might not be correct, but it is undeniable that the lesser player will miss more often than the better player. Either miss a pot, or miss shape.
Look at it this way. Let's say you are a potting god for balls 1 through 8, meaning you can pocket those balls at a 100% percentage. But you are an absolute choke-artist on the 9 and could only make the 9 ball 50% of the time. Your opponent, on the other hand, shoots every single ball at 50%. It doesn't matter if your overall net potting percentage is around 90% and your opponent's overall potting percentage is 50%, for the game of 9-ball you guys are matched absolutely evenly. On average opponent will win the amount of games as you, because every single rack will come down to the coinflip of the 9-ball.
Of course, playing straight pool or 8-ball would be a much different story.
Again, not as straightforward as you think. For simplicity's sake, let's focus only on hanging the 9 (instead of both the 8 or 9). Yes, given the same amount of chances, the weaker player by definition would hang the 9 ball more often. But again, that assumes the same amount of chances on the 9. But you would expect the stronger player to have more chances on the 9...simply because he's the stronger player. So depending on the percentages, the stronger player could hang more total 9 balls in a given match compared to the weaker player. (Again, we've all been there...on both sides!)...as smoochie has mentioned more than once, the lesser player will likely hang the 8 or 9 much more often than he/she would have on larger pockets, so it goes both ways.
To be clear, I'm not attempting to argue that tighter pockets will ALWAYS means the weaker player increases his chances of winning. Rather, I can see how it is possible.