Some consider that people who call themselves agnostics are either afraid to use the term “athest” for fear of offending, or just fence-sitters who have not made up their minds (or perhaps have not yet “seen the light” of pure atheism).
Here is an essay by John Shook (of CFI & the University of Buffalo) which discusses the definitions of these terms, both historically and in present usage.

2010/11/02 at 2:26 pm
I enjoyed his discussion of the term “dogmatic”!
I don’t really find it useful to use the term ‘agnostic’ to mean “we just don’t know enough to be certain right now.” We have other words for that, like ‘uncertain atheist,” for example. But we have no alternative to refer to someone who believes that the supernatural knowledge CANNOT be known. This reason alone is enough for me to reject Shook’s definition. If he wants us not to use ‘agnostic’ in that situation, he needs to give us an alternative.
And while this isn’t really a response to Shook, I don’t find it useful to polarize atheists and believers as though the people on either side are certain in their beliefs, positioning agnostics as the middle ground. If you look at actual people, we quickly find that such labels don’t really describe the actual continuum of thought at all.
I meet few atheists who are absolutely certain that there are no divinities. Even among the religious, I frequently encounter people who are unsure, if not about the existence of the supernatural, at least about certain aspects of beliefs required by their faiths. The more aware of their beliefs they are, the more this is the case I think (priests, for example, tend to be the most willing among the congregation to admit to the possibility that it’s all a bunch of hogwash – at least among the fliffy-flaffy ‘modern’ congregations that I’ve frequented).
That’s why I much prefer Dawkins’s idea of the belief spectrum. The middle spot is the true agnost, the person who has absolutely no opinion one way or another. Everything on the left side of that middle point is atheism, everything on the right side is theism. I can be all the way to one end, or I can be pretty close to the middle, but I’m an atheist as long as I’m on that left side.
And that’s where adjectives come in handy.