Can you really say that for sure? If the pages don't contain the exact content, it's hard to make such a comparison. Maybe the content of the pages which use -'s are structured better (or whatever google analyses for content). You must remember that people who are using -'s are probably the ones making search engine optimisations to their content itself (which probably has more effect on the score/rank itself). Until someone does a direct comparision (eg. two pages with the exact content, just different filenames, linked from the same pages, etc), or Google officially says how it works, then you'll never really know which is better.

Also remember that Google isn't the only search engine out there - maybe other search engines are penalising links that use -'s in their filenames (not likely, but who knows).

Adrian