It's only my semi-educated guess, but I believe that the manufacturers are now "binning" nearly all modules. Consider these "facts" that contribute
1) They are making thousands of assemblies, mass production at high rate
2) They must test and adjust these to comply with various power limits (i.e. <1mW, <5mW)
3) This test process is at least semi-automated and, most likely, monitors feed current as well as output power
4) Binning is based on Output Power vs Input Power (efficiency)
And, most important - the goal of the manufacturer is to maximize yield and thus profit. Any means to do this WILL be exploited to it's fullest extent. Price Wars beget other tradeoffs. This will generally continue as a downward spiral (to the consumer) as sell prices nosedive and manufacturers fight it out to maintain profitablity and market share.
Anyway, the most inefficient assemblies, the ones that need, say 200mA to make 1mW, these go into the "1mW" bin and the container ship brings these mostly to Australia, I guess.
The "average" assembly is kicked into the "5mW" bin. Most of them fit this group, as this is the largest market sector, due to FDA classifications, etc.
And the best ones - these get selected for distributors that pay a little more, have a better supplier relationship, demand better units by specification requirement, etc. And these are the "special" ones that we can get by paying a little (or a LOT) more for.
If you wind up with a "poor" or even "average" assembly, you can pot/mod it all you want, the IR laser's emission wavelength will change over temperature; the physical alignment of the crystal stack may be poor, and the doping of the Nd:YVO4 crystal might favor a center frequency on the "wrong" side of the IR LD's thermal drift characteristic (and a particular LD's mean center frequency). This is "tolerance buildup", where the parts that constitute the whole are near, at, or
beyond their design tolerances. Change any one of these variables, and the result is poor performance / failure to meet the overall original design specification.
Better assemblies are "better" because the individual optical/chemical/physical tolerances are better matched, and more tolerant of variations brought on by our meddling with pots and such.
In short, you have a dog, and you're not alone
