[rkangel]

The problem is the same as in lots of mechanical engineering designs - accumulated tolerance.

Imagine building a wall that is 100 blocks long with lego bricks. In order to fit well, the length of the first row has to be close to the length of the second row (etc.). Otherwise you will have the feeling of forcing it into place (if it's even possible).

That length difference is the accumulation of the error on each of the 100 bricks in a row so to get a good fit each brick needs to be made incredibly precisely. The required tolerance is approximately 1 micron.

Injection moulding is a surprisingly imprecise art (if you're not careful), so making them repeatably to that tolerance at high volume and low cost is a difficult skill. I can't find the reference now, but Lego has developed the skill to the extent that in the past they have been asked to advise defence companies about precision manufacture.