I have a site where all domain users have read access. When I create a
list (like a task list) and use a lookup column, select "user
information" and "display name" for my choices, I can only see some
names in the drop-down list (not all of our domain users). Why is this?

One theory I had was that the drop-down list (example: "assigned to"
in a task list) only shows me folks who have visited that site (not
the list, but anywhere in the site). Does that sounds accurate? If I
have specified users for a site (named them individually as users
instead of adding a domain group), they show up in my list- I believe
whether or not they have ever visited.