Stains from water on granite countertop? Says it's sealed...?

I purchased a granite countertop for my bathroom vanity. It says it's sealed, however, when water gets on it, it leaves a small darker stain after a few minutes. It eventually goes away when I wipe it up, but it still concerns me. I don't want to have to keep wiping it up after every use. Is it really sealed? What's going on? Thanks.