I will try it this way.
If g will find some results on amazon then this will change the results in favour of amazon ( cause goog thinks this is the best for the users ). If you add -amazon then the complete search will be traded normal.
So if in your serps nothing is changing, then this theory is supported by your observation, cause there is nothing goog can amazon favour for.
IN other words. There are different niches. Each niche is added one or more authorities. This authorities are manly display, other domains are suspressed. If you suspress this authorities in your search then the old results are coming up. Therefor it does not matter what domain you add as long this fits to the niche/authorities.
I see different results for adding different domains ( in my niche ).