Ah, that's the reason then. I have a TS256C model revision CJAA0201. I wonder why Apple has provided me a disk which is only 80 % the speed than yours. I'm sure I have paid as much for my MBA as you have paid for yours.
Revision CJAA0201 here, too.
I suspect this is a "running" upgrade, seeing as the OP has a newer machine. Note that the later Rev C MacBook Airs were equipped with Toshiba drives replacing the slower Samsung drives in the Rev B and early Rev C.
I don't fault Apple for improving the drive as production continues. Perhaps they got a better deal from Toshiba, or perhaps some better firmware came out that optimizes speed. If it's the latter, that might be something we'd be able to apply to our machines, but note that updating firmware on a drive is a destructive process (i.e. it erases everything), so it would be tricky.