I don 't think that is a bit unusual for lots of products, not just seed.
It has to do with bulk buying.
I have found that some nurseries and garden shops have seed in bulk that is much cheaper than seed in packets. That thing that turns me off from buying a few packets from a catalog, is the unreal postage they want to tack on. You about have to spend $100 to come out on the postage.
I like to look at the catalogs, but I shop locally for seed.