Knife heat treatment
Heat treatment changes the allotropy of an iron alloy. Steel doesn't equal steel; first, there are chemical differences (different amounts of carbon, nickel, etc. added) and second, there is a difference in the microcrystalline structure of the metal. 

The different allotropes (= same material in different structures) of steel have different mechanical properties. They have different levels of brittleness, flexibility, softness, etc. So, if you have two steels made from the same proportion of elements, it is possible that one of them will be harder. If you make a knife out of it, it will keep its edge for a longer time. 

Molten metal has no crystalline structure, it is a liquid. The structure forms when you cool it down to a solid state. But depending on the conditions while cooling down, different structures will form. There are a few factors determining which structure you'll get in your finished blade, but the most important one is the rate (and direction) of temperature changes to which the blade is subjected. Specific tempering processes will create different proportions of different allotropes, determining the final mechanical properties of the knife. 

I don't know which process Shun is using, but it seems that it has properties which your shop owner doesn't like. This is not necessarily a bad thing: there is no perfect combination of properties in a knife, everything is a trade-off. Knives hard enough to keep an edge longer are very hard to impossible to sharpen at home once they go dull. Knives which achieve perfect sharpness with enough care will be much worse than the average knife if you don't invest enough time in them. And so on. So, you should choose a knife based on your sharpening skills and willingness to invest time in it, not on what professionals consider the best.