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.