Thanks for the question! This is a good example of the challenges that I face when writing a book that not only has to analyze what has happened historically, but also anticipate what will happen in the future. It also shows why I am careful when phrasing with absolutes. So, let's talk about why I stated this the way I did.
Historically, Pattern games have all included Linear elements. The game I was thinking of when I put "usually" in here was the October 1994 Flasks game. This game features four numbered flasks that each contain a color, and combinations of the flasks produce different colors (for example, Flask 1 + Flask 2 = Red). However, the numbering of the Flasks in this game isn't Linear. It could just as easily be seen as Grouping. For example, they could have used random letters and it would have had no operational effect on the game (as in, Flask R + Flask B = Red).
Second, not every Pattern game has used numbers (for example, some have used days of the week) and so to avoid the automatic association of Pattern with actual numbers, I tossed in "usually." Of course, days of the week games are Linear games, but this is an example of where I want to be careful so that the point isn't misunderstood by readers. Linear includes anything that lines up items in order, but for some people there's a tendency to associate Linear with hard numbers, and putting that "usually" in there helps to reduce that.
Last, and equally important, there's no law that says that Pattern games always have to be connected to numbers or Linearity. While it certainly makes the most sense and is the easiest way for them to create these games, there are games I could imagine where they use colors like the Flasks, or obscure the relationships via directions (north, south, etc), or use groups (again, as in the Flasks game). So, if I were to say that Pattern is always Linear, it's like an invitation for the test makers to create a Pattern game that isn't Linear. I don't want that to happen, hence my phrasing here
It can be tough at times to walk the line between what's always happened in the past and what could happen in the future, so feel free to ask me about anything you see in the book. I try to be very careful in how I phrase things, and typically there's a reason behind each claim made in the text. Please let me know if that helps. Thanks!