Obviously, members of the AMERICAS group are also members of PUBLIC. So, regardless of the way the PUBLIC group was restricted, those restrictions are not to be applied to the AMERICAS group for the sales information. That group might be restricted in different ways, or not at all, and the same mechanism ensures that happens while PUBLIC restrictions are in place. Figure 12, Overriding Constraint on User/Group shows this when using the sample Interactive Reporting document, row _level_security.bqy.
This only overrides PUBLIC constraints for this particular column. Restrictions on PUBLIC against other columns are still enforced against members of the AMERICAS group as well. If the restriction is on all columns of a table, designated by an asterisk, the override must also be specified with an asterisk and then specific column constraints reapplied to groups as needed.