Anyone else see something wrong with this picture?
Not really.
So advertising 10x10 boards as costing "from $11.90", then actually charging $11.90 + $3.00 = $14.90
doesn't seem a little deceptive to you?
Granted, "deceptive" is in the eye of the beholder - which in this case is me. Not saying they do it deliberately, and as I said I'm willing to cut them some slack for various reasons, but in most of the rest of the world it'd be called "bait & switch" or "misleading advertising" regardless of the reason...
What would the alternative be? 2 separate products for 5x10 and 10x10cm board sizes?
Why not? That's what several other places e.g. iTead, Seeedstudio, etc. do.
Then people like the TS will start creating posts about not knowing they could order a 5x10 when they are looking at the 10x10 version.
Now it's shows them in one simple option with clear pricing.
I think in this case it's actually more confusing. They advertise 10x10 boards for $11.90, but for that price you can only get a 5x10 board. If I want a 5x10 board, I'll go looking for the size they do that'll fit that (like I did for my 5x8 board).
Amusingly, there's currently a graphical ad on their pages that mentions 5x10 'color free' - a size they don't otherwise list - for $12.90, with 10x10 - a size they do list - at the "real" price of $15.90.

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alternative link in case it disappears quickly)
Get over it, it isn't a big deal.
There's not really anything for me to "get over". As I hinted in my comment, it's no skin off my nose - I decided to order elsewhere using a place that gives a 5x10 option, even though it cost me what a 10x10 board would from Elecrow, simply because I didn't like the appearance of misleading advertising (and, I admit, the dodgy non-response from their payment gateway, which meant I had to wait until this morning to check whether my payment had gone through or not).
If they can ever get around to do what they advertise for the price they advertise, and they're cheaper than the options I know from experience to be good and reliable, then I might order some boards from them.
Until then, I thought it reasonable to remind people that what they advertise isn't what they deliver.