You can't be both..you can be Agnostic..and then go full Athiest..but you can't sit on three sides of a fence...

My 2 cents: the website allows one to select as many of the options as one wants. Me? I don't care much for labels. If it involves religion I'm anything I need to be to move away from it as soon as possible.

People can be fuzzy on their own understanding and application of the labels, especially given that "atheist" has a pretty negative social connotation, particularly in the US, and so people often want to avoid self-identifying as atheist even if, definitionally, they very much ARE atheists.

However, here's how the meanings of the words play out:

God -- for purposes of the other definitions we assume god = a supernatural, conscious intelligent agent responsible for the creation and maintenance of the universe.

Agnostic - one's knowledge stance about god. Do you claim to know that god exists? If you believe this is not knowable, then you're an agnostic, as the word was defined and intended by its originator (Huxley). A fairly common modern devolution of this term is one who thinks it might be knowABLE, but is definitely unknown to the person at this time. However since supernatural beings and realms are inherently unfalsifiable you can't make a defensible knowledge claim one way OR the other -- god is not knowABLE.

Atheist - one's belief stance about god. An atheist sees no valid reason to afford belief to god, and so does not. It is the absence of belief one way or the other.

Atheism tends to be informed by agnosticism, but they can (and do) vary independently.

It's a common corruption of the definitions to see atheism as DISbelief in god (when it is NO belief -- a philosophically important distinction) and then to see agnosticism as some sort of "I'm not sure, maybe" sort of halfway point between theism and atheism. This is how some people see themselves, and that's fine if they choose to, but it does not accurately reflect the historic philosophical definition of the terms.

probably because no one can prove anything 100% critical thinking whilst great and the only way to go does leave you in an awkward spot when you are an atheist but know you can't say well im 100% sure

Mostly this is because of a burden of proof. Since there is zero proof either way I can shift the burden of proof upon myself if I tell you absolutely that there are no gods. I am an atheist, yes, but I am an agnostic atheist. Both terms are one and the same unless you get technical and shift a burden of proof onto yourself. Only a fool would claim without doubt that no gods exist. Do I believe that gods exist? NO.

Most athiests are 99.9% sure there's no god's that's where the agnostic bit comes into it

JustKip has answered this most beautifully, so I am not going to add his explanation. Never apologise for asking questions, those of us who consider ourselves to be quite well educated still learn something new every day. Having an inquiring mind is what led you here in the first place!