It's not that hard to understand, but you need to get a bit of paper and a pencil.
Let's say you want the 10th to 14th of Dec off.
The Dec roster comes out on the 15th of Nov, but starts to be built on about the middle of Oct.
The plan is to have the Nov roster tie in smoothly to the Dec roster (so that, for example, the 5th of Dec to the 9th are work days).
The Nov roster begins being built during the middle of Sep, which ends up being 3 months in advance.
Makes more sense if you draw it out...