For my main site:
- Fast lists about 25% less of my site's pages than Google does.
- Fast shows 5% more back-links than Google does at this moment or 110% with double count from Google.
(but that should be at least equal as soon as the new Google update kicks in)
- Fast shows many Dmoz clones in their back-links. Google pretty much cleaned that up to zero as far as I can tell (could that be a reason DMOZ counts heavier for ranking in FAST?)
- Fast luckily shows any back-links it has indexed (as far as I know). Google above PR4. So Fast should show more back links with an equally big index ;).
I am very glad Fast shows pretty much any back-link and has no threshold.
Imagine moving or renaming an url, and having to check whom to email-ask to change their link towards you...
In your case it could be that your overal Pagerank of your site has risen. In that case Google will show more internal back-links (PR4 and higher).
I would also say that Google just has a bigger and more up-to-date index.