You may see the worst-performing variants of a given model as defining the baseline, with some being above the baseline.
But it's perfectly legitimate to define the baseline at the best-performing variants of a given model, with some unfortunately being below the baseline.
Personally, I prefer to set expectations high, and define the baseline based on the best that has been achieved so far. The worst-performing variant from 2014 should still be expected to exceed, or at the very worst be equal to, the best-performing variant from 2013.
You only need one sample to make the point that it may be slower, and that's very much contrary to expectations.
2013 models had drives from different manufactures and some of those were slower than others.
Here's an example: Say 2013-A-SSD > 2013-B-SSD and we have the same two A and B SSDs in 2014 machines.
Then 2013-A-SSD > 2014-B-SSD.
With just one machine (and without knowing the specifics) you can deduce from the above that "2014 drives are slower than 2013s" when in fact they are the exact same 2 models that 2013 Air's had.
I think what you mean is "You might get a machine that's slower than _a_ 2014 model". Assuming every 2014 model performs at the baseline, unless that baseline is lower than the previous generation's you're still getting better performance than the _official_ 2013 model.
If I have a 2013 MBA I expect the SSD when I upgrade to a 2014 one to be at least as fast as the old one.
It just tells you you'll get a speedy SSD of X capacity (true), and you're not supposed to obsess over manufacturer and a couple percentage points of performance. Else, you wouldn't have bought the Air, but some custom rig.
That's a perfectly reasonable expectation to hold, as well. One really shouldn't end up worse off (for whatever reason) merely by buying a more recent model, especially when it's a computer that's being purchased.
http://blog.macsales.com/19008-performance-testing-not-all-2...
Ah. Why would they do that? Someone could spend $3000 on MacBook Pro 16GB + SSD highest spec and risk the chance of getting bad parts? I never knew this was a practice. Is this even common in the industry otherwise Apple?
Apple has done this various times in the past, like shipping a 16x DVD drive when they advertised 8x. More problematic was the rMBP where LG LCDs were noticeably lower quality than Samsungs. http://9to5mac.com/2013/03/20/macbook-pro-with-retina-displa...
http://www.marco.org/rmbp-irtest.html
I am not sure which one of two manufacturers display is faulty, but per discussions on Apple's support Communities (https://discussions.apple.com/message/18669644#18669644), rMBPs carrying display panel from LG tends to suffers from this problem more commonly compared to those from Samsung.
EDIT: Just ran this test on a retinaMacBookPro to determine if I have a LG or Samsung Panel and found it to be working.
http://geeksblogs.com/lg-or-samsung-heres-a-little-guide-to-...
EDIT: I mean about the replacement part. Sure, they'll probably replace it after a several back and forth, plus the replacement will take serveral weeks.
If your friends aren't seeing noticeable problems though, there's nothing to warn them about. It's not that one display is inferior to the other in terms of color accuracy or brightness or quality. One display (I think it's the LG) has a noticeable ghosting and bright pixel problem in some cases.
It also lets them have a stronger negotiating position to drop a crap supplier. They can only maintain that negotiating position, though, if they buy drives from all of them; if they only used drives from whoever was the at-the-time "best" supplier, then if that supplier started to suck, all the others would demand a king's ransom for the "privilege" of switching to them.
All the component manufacturers that supply Apple are also supplying other contracts.
An example is the high-definition screens; 'Retina' Apple laptops use interchangeable screens from Sharp, LG and Samsung as availability permits. There was a run of returns last year due to some issues with the LG screens.