Because there are really, boiled down, only three possible stances on the belief in god; belief there is one, belief there is not, and lacking belief either way, a rather useful set of terms developed that provided much clarity in communication for many years. The term "atheist" was used for people with the belief there is no god. The term "agnostic" seemed suitable enough for people who "lacked" a belief either way. People who believed in a god were called "faithful" whatever god they believed in. Lately the faithful are called "theists" in some settings. The usage appears identical to "faithful," and "theist" is not an adjective but a noun, which some people prefer.
As atheism gained sway, especially in the 1950s after the production of simple amino acids by simulated natural forces, and especially in the 1980s after the common access to recorded thought provided by the internet, the search for a more respectable definition of atheism began. Many people had begun to believe that atheism was "scientific" and they did not like it being described as the "belief" there is no god. Eventually the previous order in the world described above was overthrown and some dictionaries began to list a "lack" of belief in god as one definition of atheism. A side note here, the Theory of Evolution probably did contribute to the advancement of atheism, but it was quite a slow process at first.
The first and most glaring problem with defining atheism as a "lack" of belief is that they join debates on the existence of god. To make statements while "lacking" belief is known as "lying." Of course anyone can "join" a debate or casual conversation only to ask questions and still claim to "lack" belief. That does not include taking one of the sides in the debate.
Pandemonium ensued. One good excuse for the behavior of atheists would be the establishment of the fact of the nonexistence of god. Therein lies the problem, they have come to believe so strongly that science has disproved god that they no longer see it as a "belief." But to paraphrase Daniel Patrick Moynihan, you're entitled to your own beliefs, not your own facts. They are indeed entitled to believe there is no god. That is all. Yet they continue to claim a privilege in debate they really don't have by any rules and expect others to grant them that they stand on facts, that they have "won" something they really did not. As much as they believe they are only "finding flaws in the logic" of their opponents, they are very mistaken. Their own "logic" and understanding of the process of argument is flawed.
Because such large numbers of poorly educated people still believe in their simple hearts and minds that "science" has proved there is no god the confusion of terms will continue. Some of the tamperings with definitions seen lately are creating types of atheists like "gnostic" atheists, "agnostic" atheists, "implicit" atheists, "active" atheists and a plethora of others. Some of the attempts are honest and forthright, aimed at what definitions are supposed to do, facilitate communication by providing labels that suit identifiable categories. The usual result however is that atheists are trying to claim a more "open" mind while at the same time claiming the "force" of fact.
Some of the terms are impractical. Any modern usage of "gnostic" is impractical because there is no widespread definition of what one "knows" or what qualifies one. There is no consistent widespread assigning of people to such ill defined groups. The term only has use esoterically within groups whose members have agreed to some standard definition of gnosis.
At the time of this writing the following web sites had considerable commentary on various types of atheism. Later they might become unavailable (Throw a 404 Error - it won't harm your computer, it just means "page not found") or they might change their content. The URLs are not "clickable" here so that you proceed with caution.
(1) http://commonsenseatheism.com/?p=6487
(2) http://www.atheismresearch.com/
(3) http://religion.blogs.cnn.com/2013/07/15/the-six-types-of-atheists/
(4) http://www.alternet.org/belief/6-types-atheists-and-non-believers-america
Another method of finding some of the materials is to put "Christopher F. Silver" in a search engine.
In Part II of this article three types of atheism that could more easily by used to establish consistently identifiable categories of atheists will be explored; Christian Atheism, Military Atheism, and Elementary Atheism.