I've had this happen as well. i thought this probably happens because of lack of pollination. It looks from the link below that high temps also affect seed fill, and well, we certainly have had a plenty of that.

http://www.hort.purdue.edu/newcrop/afcm/sunflower.html
Optimum temperatures for growth are 70 to 78F, but a wider range of temperatures (64 to 91F) show little effect on productivity. Extremely high temperatures have been shown to lower oil percentage, seed fill and germination.

Not sure what to think of the next quote, because it seems to indicate that for some varieties, pollination is not even necessary to set seed.

"Bees are beneficial to sunflower yield because they carry pollen from plant to plant which results in cross pollination. Some sunflower varieties will not produce highest yields unless pollinators are present. All varieties will produce some sterile seed (without meats), but varieties differ in their degree of dependence on insect pollinators. Autogamous sunflower hybrids do not require bees for maximum yield and will yield the same when covered by bags as uncovered. In non-autogamous sunflower varieties, pericarp (bull) development is normal but no ovules or meat develop. Wind is relatively unimportant in cross-pollination of sunflower. Some of the older open-pollinated varieties such as Peredovick set only 15 to 20% of seed without pollinators, whereas many hybrids set 85 to 100% seeds without pollinators."

thanks greendesert ... that would explain the flat seeds ... was wondering what made them droop?