Why Agnostic?

[Ryft]

A person can believe in God yet still have doubts, for example.

I completely agree. But therein lies the precise problem:

• "Believes in God, holds God as unprovable (or unproven) but is still 100% certain about its existence."
  

If you believe that the existence of God is unprovable, never mind unproven, then how can it be said that you are 100% certain that God exists (where "certainty is a measure of how well you think your personal beliefs represent reality")? In other words, if you consider the existence of God to be unprovable, how are you 100% certain about it? It seems to follow that if you believe God exists but have a measure of doubt for whatever reason, then you actually are not "100% certain" that your God-belief squares with reality. Ergo, '2' on your scale suffers an internal problem.

A problem that is likely created by the "certainty" use. Perhaps it would prove more useful to replace that term with something that doesn't cause such logical problems. The reason I chose "conclusively established" in my scale is because it includes varying types of argument (from evidentialist to presuppositionalist) and degrees of certainty (from fairly certain to "absolutely" certain, as your post had inquired about).

My scale is indeed too simplistic, but only by one category: it does not account for the apathetic the way yours did. And there surely are those who are apathetic, like my brother who is an atheist but never gives the issue any thought. So I would modify my scale by inserting that one remaining category

1. Gnostic (Strong) Theists:
Those who view the world (a) as though God exists, and argue that his existence (b) can be conclusively established.

2. Agnostic (Weak) Theists:
Those who view the world (a) as though God exists, and argue that his existence (b) cannot be conclusively established.

3. Agnostic (Apathetic) Atheists:
Those who view the world (a) as though God does not exist, and think such issues (b) are not a worthwhile pursuit.

4. Agnostic (Weak) Atheists:
Those who view the world (a) as though God does not exist, and argue that his non-existence (b) cannot be conclusively established.

5. Gnostic (Strong) Atheists:
Those who view the world (a) as though God does not exist, and argue that his non-existence (b) can be conclusively established.

So, who is left unaccounted for in such a scale?

The word "gnostic" means one with knowledge of spiritual matters (or one who knows God), which doesn't come across as a logical proof as you put forward.

Such issues are necessarily by logical proof, since transcendental arguments (which is how I interpretted "spiritual matters") by definition do not involve propositions that are empirical in nature.