That explanation is not really satisfactory. This blog has been in existence since +/- July 2007. Posts are archived on a monthly basis, giving a maximum of perhaps 12 archive pages. That still doesn’t get us anywhere near 500 pages; perhaps 120 at the most. Even the most cursory look at the HTML sitemap you generated will show that most pages appear multiple times in the listing.
Does this have anything to do with other sites I link to? The widgets in the sidebar?
Please could you investigate further? You may find that this is a problem affecting other sitemaps generated, e.g. other Blogger blogs.