Think of it as 8 different charts, each chart labelled A through H (the labelling is arbitrary, I think, the important part is there are 8 distinct charts). Each chart has 9 rows (corresponding to the 9 crafts including fishing), and 10 columns (corresponding to the 10 different guild ranks). This is fixed for each chart. So for instance, the Day B Journeyman Clothcraft item is a Velvet Robe. That means anytime the game picks the "B" chart (this choice is random each day, so far as I know), any Clothcrafter whose current rank asks them to turn in a Journeyman item for that day will get a Velvet Robe. These sites figure out which "chart" is being used that day based on input from members who enter information like "I'm an Artisan Woodworker and I was asked for a Maple Wand today." Usually only takes 1~2 testimonials like that to determine which chart is being used, provided people are honest in their input.
Everything else is on a fixed pattern. For instance, as you probably have noticed, you don't always get asked for an item of your current rank, but rather it cycles through every rank less than or equal to yours. This is based on how many days have passed since January 1, 2002 (that's the epoch for the FFXI clock, so if there had been guild items on that day, everyone would have had a Novice item, and then it would have began cycling from there).
So as an example, I'm veteran cooking. 2738 days have passed (in Japan) since Jan 1, 2002. So regardless of which chart is picked, my item for today is going to be an Adept-rank item. A few hours ago, at JP midnight, the game rolled a number corresponding to what sites label as "B" chart. So I get offered the B chart Cooking Adept item, which is, apparently, seafood stew.