While I bet you are really good at reading greens, this mentality confuses me (along with @iacas's previous response). Most people's reviews about Aimpoint basically say that longer putts with multiple slopes is an educated guess. Most pins are located in a spot where the slope is pretty much the same within a certain radius of the hole (that radius is often 5' or more). If you have a putt within that radius, seeing another ball roll in it tells you everything you need to know. If you have a putt outside that radius, at least you know what the latter part of your putt will do.
I should start by saying that I'm no expert on aimpoint. I took the class 2 years ago and implement it as best I can. The methods have since changed. That being said, I think think the difference is the degree of accuracy. If you're watching someone else putt, I think there are a lot of variables.
1. You're likely not in the exact same spot. Being a couple feet left or right or closer or further changes the break.
2. Are you judging their putt based on what they told you they aimed at? Where they were actually aimed at? Where they started the ball?
3. Did they play too little break but hit it hard? Did they play a little too much break but hit it soft?
I am sure that you've seen a get a read from someone else's ball then miss his putt and wonder aloud why his didn't break. Its usually because they hit the ball at different speeds or one was on the fall line and the other was slightly to the side of it.
You only get one try to focus on all of these variables--no replays, no measuring. And they all affect the putt. The aimpoint chart will tell you to aim 3" off the left edge, exactly--not a cup, or half a cup, or about 3 balls.
If we control for all of those variables, maybe you would win the 5' putting contest. But if our balls aren't in the exact same spot and I'm a crappy putter who hits one hard enough to stop 3 feet past the hole and another that dies in the hole, the information you're getting becomes much less reliable.