Whenever a wave forms through a medium/object (organ pipe) with a closed/open end, there is a chance of error in the formation of the wave, i.e. it may not actually start from the opening of the object but instead before the opening, thus resulting on an error when studying it theoretically.
Longer pipes resonate at lower frequencies, producing lower-pitched sounds.
The details of acoustic resonance are taught in many elementary physics classes.
In acoustics, end correction is a short distance applied or added to the actual length of a resonance pipe, in order to calculate the precise resonant frequency of the pipe.
A finite diameter pipe appears to be acoustically somewhat longer than its physical length.
[1] A theoretical basis for computation of the end correction is the radiation acoustic impedance of a circular piston.
This impedance represents the ratio of acoustic pressure at the piston, divided by the flow rate induced by it.
The air speed is typically assumed to be uniform across the tube end.
This is a good approximation, but not exactly true in reality, since air viscosity reduces the flow rate in the boundary layer very close to the tube surface.
Thus, the air column inside the tube is loaded by the external fluid due to sound energy radiation.
In organ pipes, a displacement antinode is not formed exactly at the open end.