My site’s RSS feed both contains more postings than the site shows, as well as less postings than are on the site. E.g. I post my Week Notes unlisted, so they do not show up to casual website visitors, but they do get distributed through RSS. Vice versa I post things to my site, such as check-ins and messages that get syndicated to twitter, that do not get added to the RSS feeds. I am not at all sure how any of those categories do or do not show up in my microformats feed. It might even be that the h-feed only shows front page postings, which means most of my postings won’t be visible in it, as I only post a selection to the front page.
The discrepancy you encounter (“items missing text”) sounds like something different though. RSS feeds might contain only excerpts of posts, where the h-feed has all of a posting (unless the blog page with also only shows excerpts). My feed is a full-article feed, but I think the WordPress default might be excerpts only.