If the locale of OS and also PHP is set to let's say german by Provider, than any values of crawling speed (priority) depends on decimal separator maintained by system.
So the values at the dropdown boxes are pre-listed as 0,0|0,1|...|1,0 but will not be qualified converted after posting the value back!
Only 0 or 1 remains so the resulting sitemap.xml only contains priorities of 0% or 100% but nothing between.
If locale is set to en_US as example, decimal separator is dot (.) and all is fine.
Could this be handled for those locales not having dot as decimal separator, because Google raises warnings, that all links get the same priority and can't be changed anymore using the plugin!
(BTW: this also affects WP 2.5.x and less)