what could explain seen ports one day, not seen the next, never seen then all the sudden seen?

ive been trying to use nesses to keep a ports list of open ports and systems
on my public network.  When I go talk to an owner of a system I am told
those ports have been open for years and they actually connect to them on a
daily basis during their daily work.  I could see an instance where a port
was seen one day and not the next then back again.  But ive been seeing more
and more ports open up as new ports but in fact have been there for a long
time.  Im also seeing instances where ports are seen during the scan, I
check later with nmap or try and connect through telnet (to tcp ports) and
its closed (most cases the ports are listed as UNKNOWN).

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