I have noticed very similiar thing. For example, I have a site, where URLs look like mysite.com/global_keyword1/global_keyword2/local_keyphrase.html
where global keywords are keywords defining the topic of the whole site, and local_keyphrase is the targeted phrase of a particular page. Deep pages typically have 12% density of targeted phrase. And they rank well for everything but their targeted phrases :))
On another site of mine, I happen to have 50% density, made mostly with alt attribute in images (it's an image gallery with category name given in each alt), and sometimes the keyphrase is also in URL, sometimes not. In first case, ranking is below #100 (nothing strange to me, as 50% density is clearly too much), but in second case - if keyphrase doesn't repeat in URL - they are #1.
What about spammy snippets - I think Google could get these sites easily - if any keyword repeats more than once in title and three times in meta, give a penalty. I'm surprised they don't do it right now.