epistable - 3:32 am on Dec 4, 2009 (gmt 0)
From my experiments it seems that advanced segments filter on the visit level, not the pageview level. So, if you filter for pageb.html, and the user has also seen pagea.html, both will show in the report. I could be wrong about this- and if so I hope someone will correct me!- but it's what I've seen in my usage.
In terms of any other way of testing the conversion rate of a specific landing page, it's impossible to speculate not knowing the details of your site/installation, but have you considered setting up a Goal Funnel or using Google Website Optimizer instead?