So here is the trick that Terry pointed me out a few months ago and Jorgen confirmed the same issue for some more agencies:
IPTC keyphrases interpreting differs between various stock agencies.

It means that if you index your image with a keyphrase like blue water, it will be retrieved OK from IPTC data on e.g. iStockphoto, but will become two keywords blue and water on e.g. Shutterstock. If you will try another trick by doble-qouting your keyphrase (note that PSM is a rare applications which allows you doing that without any problem) and your original keyphrase becomes to be “blue water”, Shutterstock will show it correctly as a blue water keyphrase, but iStockphoto will not show it at all (BTW, why is that iStockphoto.com?)

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