A Google Webmaster Help thread reports that in some instances, Google will tell you that your Sitemap file is OK, but in reality, it is not.
The issue comes up when on the summary page, Google may report that your Sitemap file is set to an "OK" status. But when you click through to get the detailed report, it will then show that the Sitemap is not OK, but rather has errors.
Googler, JohnMu, said:
It looks like you've stumbled upon a bug or something similar :-).
So, don't just rely on the the summary page, make sure to click through to see any Sitemaps errors.
Forum discussion at Google Webmaster Help.