Finder search does not find all relevant documents MacOS 10.6.8
On my system, typing successive letters always shrinks the results menu.  Have you tried rebuilding your Spotlight index?  Open terminal in an admin account and type:

sudo mdutil -i on "/Volumes/YourHardDiskNameHere"


, authenticate with your password, and go get a coffee.

Edit:
Another way to accomplish this is to open Spotlight preferences to the privacy tab and drag your HDD icon from finder onto the "Prevent spotlight..." list.  Close preferences (may not be necessary - ?). Open it again to the same tab and use the +/- buttons to delete the disk.  Re-indexing should start.