In article ,
David wrote:

That's the only explanation I can think of.

My hunch is that your hunch is right.

The router itself will exert some pressure towards the fence which will
deflect ANY fence somewhat.
I always clamp a cleat at the centre of my long fences in order to stop
that from happening.... or at least reduce that flexing.
Even then, I make a 'whisper pass' as a final clean-up so the router
doesn't have to work that hard.