I'm guessing that you're encountering what's known as "overscan"

Rather than type a lengthy explanation of what it is and why we have it, best to just read the relevant wikipedia entry:

Overscan - Wikipedia, the free encyclopedia

EDIT: Many NLEs have guides that you can display on preview windows/title generators that show the "title safe" and "action safe" areas. In other words, the screen area for your titles and footage that should be safe from overscan.