Dear PigInSpace and dear microstockgroup friends.
As somebody who have certain knowdlege about IT and how motor search engine works i can tell you that this is not something strange.
In one hand we really dont know how "popularity" search works, it is maybe related to sales and relevance; but in general an image can rank different when you search under different terms or phrases.
For example if you have a pic of a Boing 777, this pic may be the first (in first page) under the search term "Boing 777", but maybe your picture is in the thirst page under the search term "plane", and maybe is out of the search under the term "fly" because you dont use this term in your metadata.
In the other hand, if a pic sells a lot under certain term, this dont give to the image a better search position under other terms (this is speculative, but almost certainly does work well).
Again in the Boing 777 example, maybe you sell the image 1000 times under the term "boing" so it is the first in popularity under this term, but maybe you dont make any sale under the term "white", because the image have no sales under this search term.
The way that multi-terms combine is unknowed but it is a equation that link the sales of the terms used, like "boing" and "777", you have 1,000 sales under "boing" and maybe 350 under "777", other image have 550 under boing and 900 under "777", what image is first under the phrase "boing 777" ?? some logical rules are playing here, maybe the sum, maybe a more complex equation, but essentially it works like this.
I hope to help everybody understand about how search engine motors works.
Regards