Why does frozen spinach have so much less iron than fresh spinach?

Why does frozen spinach have so much less iron than fresh spinach?

I have been trying to increase my iron intake and was dismayed to learn
that my frozen spinach only has 2% of my daily iron needs while fresh
cooked spinach has about 20%. I have been under the impression that
freezing vegetables preserves their nutrients from the moment they are
picked/frozen and that transporting fresh vegetables across the country
depletes the nutrients.
There was a whole about this some time ago either on TV or the internet or
someplace.
So when I read the back of my frozen spinach container I was really
surprised to find that it only has 2% of what I need.
Why is this? Is there a difference between the kinds of spinach that are
grown for canning/fresh/frozen? Does freezing break something in the case
of spinach? Does this happen with all fresh green leafy vegetables?
Thank you for your time.