I don't know how the search works exactly, but if it is sorted just by DL #, then as long as the image has the keywords it will show up in the search in the appropriate position based on total DL #. Maybe none of the DLs came from that keyword. Also images that have been around for many years are more likely to have a higher # of downloads. Based on my own images many of the "most popular" sorts of searches are not actually ordered by the total number of downloads.
Yes, it is unclear how exactly the sorting is defined. Most likely, all images that simply contain this keyword will be displayed, regardless of their position in the keyword sequence.
It's also stupid that you can't see any keywords of images and unlike Dreamstime, you can't see which keyword was used to buy your images.
So for research purposes the sorting is literaly crap.
I would do it the other way around instead.
Pick a topic and use a meaningful keyword to see how many images are found. If it's less than 100k, better 30 to 50k then it's worth creating images here.
So narrow down the topic. For example sightseeings in Amsterdam.
Otherwise you have no chance. Or you are an extremely talented photographer and can hold up against the pros.