Have more info in the parent comment i left but to give you empirical scenarios for two fences not back to back: neighbor A put in pool before neighbor B put up fence. A’s pool fence was done to look good around pool while B’s fence installed later was done by contractor instructed to fence the perimeter.

One fence was installed diy based on an mutually implied property line. New neighbor moves in to house without fence and installs their fence to actual surveyed property line.