Yes, the environment and climate are different every year even from same location. Some distributors are very sensible to this (such as avat owner).
Some distributors ( such as NG) follows what the nature produces, but some fragrance producers will add certain chemical to ensure their perfume smells exactly same even if the original essential oil is slightly different.
If kept well, some essential oil (for example, rose otto) gets better and better and wil keep their value. But some will getting worse, for example lemon (that's why BBW has annul sale to dump out those inventories, I guess).