Old hats like Silver needle will be more eloquent and specific, but I'll try to sum up my limited understanding:
In a nutshell, they are selling out before you can complete your purchase. There are only a few knives that pop up durring Wednesday sales, and they go quickly.
I'll try to reiterate and boil down what I understand from the How to Buy pages you referenced and my own experience, in case that partly answers your question: They do not have knives in stock at this time, and you can't buy one the day that you want it. You need to plan far ahead. They are starting another product line called SK that they hope to produce enough of to maintain stock on hand. Given the demand to date, I am guessing SKs will sell out just as fast as the GSOs.
http://surviveknives.com/news/big-plans-for-the-future/
They have had pre-orders and presales. You pay up front, and have to wait until they do a production run to receive your knife. This can be many months or in some of our cases, years, depending on demand. Once the ordering periods close, Survive! produces the knives ordered. Sometimes they have extra material available for a production run and have the option to produce some extra knives. At that time, they may open up additional pre-sale orders.
http://surviveknives.com/news-information/
During production, some knives that don't meet their exceptionally high QC standards are sold as Factory seconds. If that occurs, they typicall ypost them for sale on Wednesdays. It is not a reliable way to purchase, and if you are not among the first in line, you will miss out on the very few knives that go up. (noted on the first line of their news/information page, link above)
http://surviveknives.com/knives/factory-2nds/
Once a run is done, there is a chance that some surplus Factory Firsts will also be posted for sale.
Sign up for thie First Friday newsletter and you will get an email each month stating what is happening at the company, and if there will be a chance to place an order before the next production run begins.
http://surviveknives.us7.list-manage.com/subscribe?u=eadc85765dd1915442d1d6274&id=b9305964f6
All most of us can do at this point is to stake out the Wednesday F2 sales or buy on the secondary market. If you are able and willing to wait through the company's growth spurts, which I find to be well worth the wait, place an order whenever a presale window opens up. But read up about the extended wait times to properly set your expectations. If you have a tight timeframe, presales and preorders are probably not for you.
Hopefully that will give you the broad strokes. I understand it repeats some things you may already know, but I expounded in the hopes that it may help another reader. The pros will chime in shortly and correct/get specific. There is some great expertise in this forum, well worth reading through it. GL!