The short version
A deed is a written description of where a boundary was meant to go. A fence is physical evidence of where people have behaved as though it goes. These diverge more often than anyone expects.
Why the difference persists
Descriptions get copied forward across decades. A transcription error in 1962 propagates into every subsequent conveyance, and nobody notices until two neighbours both mow the same eight feet.
What a survey actually resolves
The surveyor retraces the original monuments, weighs the evidence, and reports what the boundary is — which may or may not match either the deed or the fence. Read the services page for what that involves.