There's a few weird things about class a designations, one being that it seems to me like it's a lot easier for a tiny creek to be class a. I realize the designations are based on pounds per acre (actually kilograms per hectare), but if you think that 100 to 200 yards of Penns Creek would need to be able to support 35.6 pounds of brown trout to be class a wild brown, you can follow my line of thought. You'd be hard pressed to come up with a full acre of surface area (40,000 square feet) in the whole of East Branch Antietam above the Waynesboro reservoir (the class a section). As a practical matter, if you presume the creek to be 10 feet wide, you'd need 4,000 feet of stream (three quarters of a mile) for there to be a full acre.
Suppose, for the sake of simplicity, you presume (not unreasonably) the creek as having a full acre in the class a section, that means there are at least 26.7 pounds of brook trout in that section of water (the class a threshold for brook trout is 26.7 pounds per acre; for browns it is 35.6 pounds per acre and for rainbows it is 1.78 pounds per acre). Presuming you need a sustained brood stock of fish in the size range of 10 or more inches long, and guessing that a reasonable brood stock in that small section of water would be 30 fish at an average weight of about about a third of a pound (5.3 ounces) each, all of a sudden you have accounted for about 20 percent of the total biomass of the population. Add in a few hundred one- and two year old fish and, all of a sudden, you are at the 26.7 pounds of brook trout you need for that tiny creek to be class a wild brook trout.
This extrapolates to the reality that if you are the second person to fish the creek in a single day, a fair number of the fish have already been spooked once, so catch rates will go down. Also, on a tiny creek, it is possible to do a tremendous amount of damage to the brood stock by killing just one or two of them, either by accident or for the frying pan.
Meanwhile, Penn's Creek is 75 to 100 feet wide, so, using a middle ground number of 85 feet, you only need 470 linear feet (150 yards) of stream to complete an acre. It is very easy to imagine well in excess of 26.7 pounds of brown trout in that much of Penn's Creek. And you'd only need two or three big rainbows and a handful of one and two year old rainbows to meet the Class A threshold for that variety.
I guess the short version is the smaller the class a stream is, the worse the fishing (and the greater the chance of damaging the population). Also, class a rainbow streams are essentially the same thing as class a unicorn forests, at least from a fishing perspective.
You can see the class a, b, c, d and e breakdown for Pennsylvania for brook, brown and rainbow at this link:
I don't know about any of you, but that's more math than I like to think about.