Try this logic - if you are a product designer, and you know your widget changes its characteristics from its new state to its well broken-in state, do you optimize its performance for the new state, which exists only for a short time, or for the broken-in state, which lasts a long time? I think any intelligent designer would do the latter. I think this explains why some products work a little better after being broken-in.
Break-in
I did a quick search and didn't find anything that specifically addressed this question: Has anyone ever found that a product actually sounded worse after it was used for whatever break-in period the manufacturer and dealer recommended? I mean, doesn't anyone find it odd that components always sound better? It could cause a person to wonder whether, to some degree at least, some of the "break-in" is happening between the ears.