my slant buildup

I do have a question now that I have enough stuff here to actually finish assembly...
When I get to setting the valves, Oregon Cams had a tighter spec than OE. It's still a factory /6, just a freshly rebuilt one. Thermal warming and cooling should be the same as stock, maybe a bit more "heating" with the block + head shave to bump CR. My cam is still the stock cam it left the factory with, just reground. How come the lash wouldn't still be as listed "stock"? Not looking at the cam specs right this second, but I do know they list "desired lash" the same, on both intake/exhaust. I believe the number is 12-thou. Again w/o having that sheet right in front of me. using all stock/OE original pushrods and rocker assemblies.
How/why are they spec'ing a tighter lash on the exhaust? I would think that I'd want to keep that as-factory-stock. Why wouldn't I?