Well, well, as you please. All flying-school or rental aircraft that have managed to pass an annual inspection, with compressions probably in the low-mid sixties, draggy and leaky doors and seals, and props that may or may not be identical to the one used for the model's original certification test (or may or may not be out of balance, and may or may not have been filed down to limits to eliminate nicks) may always be depended upon to turn in the performance figures contained in the POH.
Good to know. Back in the real world, some points to consider:-
1. For aircraft manufactured in the U.S., book performance figures are calculated by the manufacturer at the time of certification. They are notorious for erring on the side of optimism. Barry Schiff, formerly of TWA and Flying magazine, has a good piece on this in one of his "Proficient Pilot" columns.
2. Until March 1, 1979, POHs were not legally required to be provided at all. That date cut-off includes a lot of aircraft currently flying, including the one I own.
3. Most pilots do not include in their pre-flight check many things that can affect performance, e.g. tyre pressures, or whether the RPM figure shown on the tachometer actually represents how fast the propeller is turning (hint: it typically isn't).
4. The FAA points out that each individual aircraft will differ in its flying and performance characteristics.
5. U.S. AOPA's Air Safety Institute recommend that pilots flying light piston aircraft factor in a safety margin of 50% above book performance chart figures for takeoff, landing and rate of climb.
6. Mathematically, it is improbable that all pilots are "of average skill." If they are, it is improbable that their own performance on any given day will invariably be at or above average.
As you were, gentlemen. Keep chasing that sizzling 175 fpm promised by the book—I'm sure it'll always be there whenever you need it!