Integrated circuits are manufactured within tolerances.
1) The chip is bad and discarded
2) The chip is partially functional, they disable the non-working sections and sell it under a different model (GTX1080 vs GTX1070)
3) There are subtle differences between any two completed integrated circuits. The voltage required to reach certain clock speeds, power consumption, etc all differ.
The key thing to note is that the manufacturer will have a set of tests they put each integrated circuit through, if they pass they are shipped off to be assembled into finished products. GPU vendors may have their own tests and will perform what is called binning. They will find the chips that run at the lowest voltages (usually end up in laptops for lower power consumption), those that will clock the fastest, etc. Then they will make their different models available with silicon that meets the profiles they have created. The longer this process takes the more it costs, so you do pay a premium and you get more than one product for each given GPU line.
Personally I would get the EVGA, but only because I like the concept of the ICX tech. Looks pretty cool too. I've had a few ASUS GPUs, even the early versions of what became STRIX. When it comes to GPU shopping, really comes down to cooling capacity and appearance. Both GPUs are fine in both categories to me. Pick the one you like, or the one that is cheaper.
Pascal GPUs tend to have the same rough overclocking limitations and boost profiles. So no really need to worry about a few dozen megahertz either way.
To get super specific about integrated circuit manufacture:
Raw materials: Silicon crystal. You take as pure a silicon as you can make, melt it down, put it in an iridium crucible, and let it cool so that is forms a perfectly uniform surface.
Then you slice it into wafers.
Dope them with layers of different semiconductors.
Lay a mask to create a pattern.
Use a laser to etch into the layers and produce the profiles associated with transistors (And lately, repeat a few times to produce more 3-dimensional profiles)
Now every machine doing this work has tolerances and sometimes the result isn't perfect if all the tolerances swing one way, you end up with non-functional hardware. Those chips that meet or exceed all the expectations and work flawlessly sometimes make it into products you can buy. Some circuits are only tested, not binned. Not doing that is cheaper, so you can occasionally end up with a really good CPU or GPU that will run fast and cool. This is known as winning the silicon lottery.
Keep in mind that you are trying to get millions of transistors to all work at the same speed. Only takes a few bad ones to make a chip unstable. It is a miracle they work at all.