That is very good question. I was thinking about same things last few days because my favourite Aqva changed into something I dont want to wear anymore.
I think it has something to do with:
* distance you spray from
* amount you spray
* your mood (the way you perceive things)
Also it can be that every 0,1ml of fragrance is not 100% consistent.
Just my theories because I dont know *** about that