Or is it just sloppy manufacture?
Partly that, and a "why worry, people are buying them like hotcakes," attitude. I suspect that many of their suppliers are either working to their own prints, or no prints at all. No one is spending big money on inspection/quality control either. The guns and gun parts that Uncle Sam bought were inspected not only by the contractor's inspectors, but those employed by the government working in-house too.
While this mixture of dimensional standards might be true across the board, it is most evident in the lower priced lines - for obvious reasons. The basic design of the 1911 platform is not one that does well with cost-cutting modifications, but cost containment is the prime motive that drive gun makers.
If you buy a Glock, SIG, Ruger, Beretta, etc. you get a pistol that's made by one manufacturer working to their own blueprints and standards. If you purchase a 1911 platform pistol you get one that's made by numerous companies, most of which simply assemble a product out of parts bought from outside suppliers. Each of these part vendors and the "gun assembler," is working to their own standards, or lack of same.
Some makes and/or models are inexpensive, while others are very much so. But it's unlikely that a lower-end MIL-Spec model will be equal to a top end one, gadgets, gimmicks and cosmetics not withstanding. A custom pistol build by a master pistolsmith such as RogersPrecision will be a far cry from a S.A. MIL-Spec, but it will also cost three times as much or more - for good reason.
The bottom line is that the less expensive in relative terms the pistol is, the higher the chance of getting a lemon, although not all S.A. pistols meet this description. Obviously a high percentage of them do work, at least for a time. But this doesn't provide much assurance for those that get one that doesn't, or are looking to invest more money to improve their gun with aftermarket parts, etc.
And yes, on this forum we have explained this over, and over, and over.... :banghead: