No, they aren't.
The POH data uses as near as can reasonably be achieved accurate factors, then the 1.33 / 1.43 factors are there to take out inaccuracies and variations in flying technique.
How accurately the numbers are genuinely adjusted from standard values will tend to depend upon the value of the aeroplane - Piper are middle of the market so tend to do a reasonable job, but I'd expect much greater effort on something like a Citation than a PA28/C172, and it to be pretty crude and basic on something like an LSA.
G