Are you sure your card isn't fake? A lot of 3rd party sellers are selling SD cards and USB flash drives with fake capacities. If yoy bought it from a reputable retailer then this shouldn't be a concern. Try running H2testw to determine if your card has a fake capacity:
http://www.softpedia.com/get/System/System-Miscellaneous/H2testw.shtml
It was a verified purchase from Amazon. Still, it wouldn't hurt to run the tool and see.