I've never bought minis, so I can't comment on that part. The sample vials that I've gone on to buy full bottles of I found to smell quite the same. Now it is quite possible to get mis-handled scents and for that to be the problem. You didn't mention where you have been getting your samples from, perhaps that would be a key piece of data.
I know I wonder a bit about the department store testers that get low on juice, are very clear bottles and are right by the door where sunlight streams in. A sample decant from one of those would be suspect in my mind.