I see 13 m2 kites, it all depends on how the series are build from different brands.
Dont think anyone omit a 13 because of the "number" itself - have never seen anyone offering kites in a series up to big kites, and then no 13 m2.
Can not see it matters though, as if you are heavier a 14 m2 can do instead of a 12, and the brands usually "fit" their spacing so you can get a nice quiver.
Some brands got a spacing like (4-)5-7-9-11-13(-15) instead of (4-)5-7-9-12(-14) which is also good IMO if you want few kites.
Those having 1 m2 spacing is of course a luxury for us as users, but not many of these for obvious reasons, and as we are so stupid as customers, that we want new kites "available" so often, that making a good series with small spacing and keeping it for 2-3 years is a no go.
Eventhough I would normally prefer that, much better and I prefer 1 m2 steps between sizes in the small and medium ones, so I wear half as much on my kites, can take another one if one gets destroyed on a perfect waveday, and I can choose exactly the size I want on a given day.
From 5 to 6 m2 is 20% bigger and a nice spacing as 5 to 7 is possible but not just as good if you use these sizes often (I do).
Where you from 12 m2 +20 % go to 14.4 m2 which should work well, and this is my point, in these sizes one can live without small steps in my experience, no matter what you weigh.
Regarding offering new kites more rare, and build the series "better" and more sizes instead, the thing is these last 1-2 years, with hydrofoiling exploding now, there is a big market for lighter bigger kites, so a quantum leap is happening in this respect now, and as they also work okay for light wind waveriding, things change fast right now - so maybe I dont like sticking at good solutions for longer right now anyways ha haa
Peter