Thanks for your response Ron but I'm still puzzled. I think it might be linked to which web browser is used.

I've just tried a search on Shutterstock for "flower" and then "flowers" using the top three browsers - Internet Explorer, Mozilla Firefox and Google Chrome. All produce the same number of results for "flower" and "flowers" but they differ as to the sequence of images.

On Internet Explorer the sequence of images is identical for both "flower" and "flowers" (and for the search words tried previously). On Firefox and Chrome they are different. And all three produce different sequences from each other for the same word.

Since Chrome seems to be the most popular browser at present, followed by Firefox it would seem that the use of singular or plural in searches on SS is going to yield different results for many people ... and presumably there's nothing we can do about it in our keywording.