I'm not a mechanical design engineer, but I'm guessing that you can only get that big an error if the design was by a goof-ball know-nothing. Shouldn't the design keep that much error from occurring? I'm thinking it should be impossible for a good design to screw together and turn out like that. No?
Yes, a good engineering design takes the burden off of the downstream operations, whether they are a manufacturer, installer, or consumer. In the case of the pocket castings being so far off for decades, I can only "guess" what is going on. My guess is there is a lot of manufacturing variation in the parts, so BW designed a lot of play in the components, and left it up to the installer to align everything better during the install. Some installers do it well, some don't. That "could" explain it. It could also be a case where there is so much manufacturing variation, and the design did not allow enough adjustment for the installer to align properly. That is another possibility. Probably several more possibilites. Full disclosure, I have not taken mine apart to see what is going on.
Now, in decades of this happening, and no improvements in all those decades, you would have to blame the engineers, not the installers, even if the engineers left enough play for the installers to get it right, IMO. The engineers should have seen after all these years, so many pockets were misaligned, and their design did not work. They should have "designed it out".
Diamond on the other hand, did "design it out". They knew of this decades old problem on the GC. Their tables are dry assembled at the factory with dowel pins at each pocket. Then after assembly, blending can be done where the pieces meet if necessary, to overcome any manufacturing variations. Then the table is disassembled. Now when it gets to the installer, he simply assembles the table, and the dowel pins maintain the alignment. That is an example of the engineers "designing out" the problem. But what is the cost of doing this? Extra time, extra labor, extra floor space, extra money? In exchange for a higher quality product delivered to the consumer. Is it worth it? That is a business decision.
To take that further, I believe diamond numbers each connection point, to keep that alignment if any blending was done. Now that is ok, but that could still be messed up by an installer who didn't match up the numbers. In that case, there is a saying, "there is always a bigger idiot", and you can't design out all possibilities. But, to make it better, the dowels could be different diameters at each corner, or different spacings. This would prevent mis-assembly of the corners, even if the person didn't see the numbers. You can take this as far as you want. But there comes a point where its too far, and the product become too expensive or too complex.
I was designing swings that rock babies to sleep for 15 years. We would have parents come in and put them together following the instructions. It was a shit-show. That really showed the engineers how difficult it can be for people, and how to improve. I suspect BW either doesn't care, or doesn't know, or doesn't want to spend the time/money to improve the design of decades of these pocket castings sticking up.
For as much as I like GC tables better than Diamonds, and how they play, I am disappointed after decades this is still a problem.
TFT 
