Invalid link to /blog/ show blog rather than 404

forgive me if this has been discussed but can't find it anywhere on the forums... on my site if you goto /blog/any-sort-of-string.html it will return the blog homepage even though "any-sort-of-string.html" is not a blog post/alias. Why doesn't it return a 404? I have custom 404 setup and works fine with this one exception. anyone have any ideas why this would happen or at least a place to start to debug?

Thanks.