For right now, I suspect the increased % of sales from Tablet customers is mostly the early adopter effect. People who have tablets tend to be in the early adopter category - whether it's because they have the disposable income, because they're the type that always has to have the latest and greatest regardless of the effect on their pocketbook, or they're in a field where having the latest computing technology is advantageous.
Any one of those three things describes an A+ consumer, that a savvy retailer would cater to.
I get a chuckle out of the naysayers, partly because I was one when it came to tablets for a fair while. I just couldn't see the point. Then I ended up with a pile of bonus money and decided what the heck and picked up an Android tablet, fully expecting it to annoy me. It didn't. It has replaced my need for a laptop. I've had a half dozen or so laptops over the past decade (usually paid for by an employer or as a part of a contract), and I always considered them nothing more than a necessary evil - too small and underpowered to be actually useful, to bulky and heavy to be truly portable.
Now, I take my tablet with me most places. Small enough and light enough for portability. Large enough viewing space to accomplish light tasks that I would do on a laptop (I got one with a 10" screen). Nice for watching videos or reading a book while I'm killing time, and unlike a laptop, I can watch SEVERAL movies before draining the battery (important if you find yourself stuck at an airport).
Not only has it replaced my need for a laptop, I'm actually thinking of getting a "dumb" phone with long battery life - so long as it has 4g and BlueTooth for tethering. I hardly use my smartphone at all anymore for checking email on the go or other such basic web surfing. I would rather have a phone that I don't have to charge daily that serves merely as a phone and a connection point to the web for my tablet. It's FAR better than any smartphone could ever be at that kind of tasking.
Anyway, back to the point at hand - the buyer effect won't last for long. Maybe another year or so until they become more mainstream, and then it will likely level out to similar levels as other devices. But to ignore the form factor when you're designing your site is silliness. It would be the same type of silliness as designing a site that only works for Internet Exploder.
So, if you're smart about design, you'll simply add in common browser tablet clients to your usability tests. And if you're even smarter about design, you've been using liquid layouts and common standards since the turn of the millennium, and your site will look presentable (if maybe not perfect - at least presentable and usable) on Tablets, Smartphones, and any Browser/OS combo your visitors are likely to be using.
I've got a half dozen sites on the go. I haven't had to redesign one of them, not one, for smartphones or tablets, because I took the time to build them right the first time, and they all have accommodated the new form factors just fine.
Not bragging about that really, because if that ISN'T how you design your sites, kindly log off the web, you're cluttering it with crap.