It's all a matter of bucket availability. You have several fare buckets on economy: Y, B, M, E, U, H, Q, V, W, S, T, L, K, G, N, with Y being full fare unrestricted, and N being deep discount economy. This means that N fare will be cheaper than the others, but will have more restrictions, including advance purchase date. Each fare bucket is allocated a number of seats, and when those seats run out, you move to the next more expensive fare bucket. Meaning that if N runs out, you will only be able to purchase G+. When G runs out, K and so on.

As I mentioned, each fare class has its restrictions, which include advance purchase date. It's very common for it to be 21 days out for the cheapest fares. If you go through the purchase process, when you get the flights summary and it displays the final price (before checking out) you can "View fare rules and restrictions". There, you can see if there are any advance purchase restrictions. However, dedicate some time to reading those, because they can get complicated.

If you want to check fare bucket availability when shopping for flights, you can enable Expert Mode here: https://www.united.com/web/en-US/app...reference.aspx

Go to the bottom of the page, and enable it. Then, when you search for you flights, just click on Details for a flight, and you will see the fare buckets listed at the bottom with its availability. Bear in mind these are just a reference and may have some delay and show 'ghost availability'. Also, just so you know, Y is the most expensive for Economy, anything before that is Business or First. X and XN are reward buckets. And the numbers won't go up more than 9 - that means there are 9 or more seats available on that bucket.

Hope that helps a bit on understanding fare buckets and pricing!