Frozen Broccoli Floret Nutrition

Maybe they are chopped finer so you get more in a cup. Probably not enough difference to worry about.

Well for example the vitamin c is drastically different.

http://nutritiondata.self.com/facts/vegetables-and-vegetable-products/2356/2

This may account for some of the difference:
http://www.whfoods.com/genpage.php?tname=george&dbid=215
"About 25% of the vitamin C and a greater percentage of folate are lost during the blanching process that occurs before foods are frozen."