boB,

I'm only using one system now, but in the past I have used two different
systems when accessing messages.  In one of the  r  replied to the list,
  R  replied only to the sender, while on the other system it was the other
way around, as is the case with the system I am on.  So if I don't pay
attention and/or don't remember my reply won't go where I intended.

-glen-