Rails - Oct 2014 - before_update callback is called when new record is created

All my tables/models have a "revision" field, and I have a
before_update callback to increment the revision field.

Among my tables are:
  events
  rooms
  events_rooms

In my Event model, I have:
  has_many :eventRooms
  has_many(:rooms, :through => :eventRooms)

When I create an event, assign a room or two, and then save, the
"events_rooms" table entry is initially created with a revision of 1,
and then (immediately, automatically, and inexplicably) updated to have
a revision of 2. I do not want every new record to have a revision
number of 2.  The revision 2 version is identical to revision 1 (which
it replaces), aside from the revision number.

I can reproduce this in the rails console with the following:
  event = Event.new({'name' => 'Meeting 5'})
  event.rooms << Room.find(['1', '2'])
  event.save

Does anyone have any idea why this could be happening or where I should
look for further troubleshooting?

I'm working trying to strip out all the weird stuff (there's a lot) that
I had in my app, and remove anything proprietary so I can make a simple
test case.
So far, none of my simplifications have made this problem go away.

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