The best match I can come up with between what I see when trying to search, and what the code may be doing on the back-end is this:
Parms received from browser.
Some filters are applied to form a very broad query.
Results are capped at x records.
More filters are applied.
Consequently, your post can be cut off by the initial cap before the query limiting filters are properly applied.
This happens because the pool of posts to search against is artificially trimmed too early in the process, and is a very common mistake. Also, a hard one for a developer to catch without testing it on a fully populated system. It will always work right with say... less than 200 posts. It will in fact USUALLY work right with a higher number of posts. However, the higher the post count in the DB becomes, the more this error condition comes into play.
If you want me to take a look at the stuff, I can. I've only done one LAMP contract before, but it's a pretty basic system so I should be able to figure out what's going on.