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

Judging by the nutrition information I can easily find (for example, from nutritiondata.self.com), frozen spinach is generally cooked by boiling and draining. That's pretty much what I'd have guessed; it's certainly the easy way to cook things.

Unfortunately, that means that some nutrients are lost with the discarded water. I doubt it'd be different for any other green leafy vegetable - though it's certainly much harder to find frozen mustard greens!

The nutrition information for fresh spinach is for fresh, raw spinach; if you boiled and drained it, you'd make the same sacrifice that the frozen spinach has.

As for your generalization, freezing vegetables and fruits does preserve most things pretty well - but only what's actually left in them when they get frozen! And while transporting fresh vegetables around can cost you some "freshness", it's not going to affect minerals. Some vitamins could be lost by breaking down (I'm not an expert here, but it seems possible) since they're more complex molecules, but minerals are just single elements. Those iron atoms won't fall out of the spinach on the way, and they're certainly not going to be transmuted, either!

Edit: Essentially what's been said in the comments is that the USDA nutrition facts say that "Spinach, frozen, chopped or leaf, cooked, boiled, drained" has about half as much iron as "Spinach, cooked, boiled, drained". A discrepancy, indeed, though not the same one cited in the question. My interpretation here is that the frozen spinach is simply not cooked in the same way (boiled and drained more thoroughly) as the non-frozen.

An alternative, proposed by Adisak, is that the frozen spinach nutrition means that you've taken the already-cooked, frozen spinach and boiled it again and drained away even more nutrients. This seems unlikely; frozen spinach is already cooked, so there's no reason for the nutrition information to assume that you'll boil it over again. The description (it seems to me) is referring to the cooking that took place before it was frozen.

Spinach is an okay source of iron as long as its not raw or boiled. Boiling, obviously, causes it to loose too many nutrients (I wouldn't boil any vegetable for that matter) and like stated above, the raw form is high in iron but not fully absorbable by the body. Fresh spinach, either lightly steamed or lightly sautéed is best. The same is true for other leafy greens, ie. kale and Swiss chard. Also, vitamin c will help your body absorb the iron from the spinach.

From Eating Well:

*Fresh Spinach vs. Frozen Spinach**
There’s a reason Popeye reached for a can of spinach rather than a fresh bunch. He knew that he could get more bang for his buck. You can squeeze a lot of spinach into a can or a box, delivering more spinach in less volume. (You would have to eat a mountain of fresh to get what you can in a 10-ounce box of frozen.) We prefer frozen spinach over canned—it’s got better flavor and is lower in sodium—but the same principle applies. One cup of frozen spinach has more than four times the amount of nutrients, such as fiber, folate, iron and calcium, than a cup of fresh spinach, so if you want to power up, do it with frozen spinach.

Winner: Frozen spinach!