Segments work very differently than filters. Segments are based on containers: hit, visit and visitor.
You are using Visitor container. Assuming you are using the segment in Pages report, ALL pages from visitors who saw "campaign contains affil" (and not just the hits/Pages where the campaign was affil) will be shown in the report. While filter will give you the page names, only where "campaign contains affil".
More details on segment containers and how they work can be found here.