Data for the front page link are changed even when not necessary

The front page data are changed even when it should not be necessary.
Supposing that xmlsitemap_output() gets called 2 times in 10 minutes, and that in both the calls the sitemap needs to be updated, the front page will get a different value for $row->changed, which is in contrast with the value used for the front page change frequency that could be 3600 seconds.