I think that, basically, the convictions are the same. Where there is more hostility
to the non-religious, some atheists adopt the agnostic tag, since it seems less hostile
to religion and less likely to get them into trouble. For the same reason, they will probably
need to justify this decision more, pointing out that they are not as big a danger as
atheists. Atheists who claim to be atheists, on the other hand, obviously don't care about
such sentiments, and so obviously don't feel any pressure to justify what they call themselves.

That's the difference.

Of course, whether or not one perceives hostility is subjective, so even in the same society
there will be some atheists who prefer the agnostic label.

It's not about believing something doesn't exist vs. not believing something exists and it's not
about being sure or not.