Speaker Break In...? Or listener “Break In”?

Why do some driver manufacturers recommend a "burn in" period. Say, 20 hours continuous music a low level?  Varnish on the cone? 
My own experience in 30 years of working on professional broadcasting equipment, is that the ear adapts to accommodate different loudspeakers. 

Somenone says the sound improved after two hours I might believe. Someone says the sound improved after 400 hours I wouldn't. That's just sales talk.

So annoying to hear categorical claims such as 'all do' or 'all don't.'  Does it not occur to the people making such claims that 'some may' and  'some may not?'  Like many claims of physical characteristics in audio, these should be amenable to physical measurements. I spent 5 years as the "human factors" postdoc in a department of electrical engineering and saw this being done all the time. Are there no real engineers in audio?

Notice I don't say measurements that people can detect any difference.  That type of measurement is much more difficult to carry out given the inherently noisy nature of human testing. 

I agree with everything prof1 said. These things have been physically measured. The changes that occur are quick and inaudible. It's listener break-in.

Gosh, do you think maybe the changes are inaudible because the people listening are hearing challenged? Or because they’re so busy making changes any changes that do occur are hidden in the noise? Hel-loo! Physically measured? By who? Nobody ever measures break in. You don’t even know what to measure. Give me a break!