Maybe the search engine considers the addition of keywords as something bad, and the removal as something good. It makes sense when people submit with few keywords to avoid a rejection due to them.
If the perception that adding/removing keywords affects the best match placement is true, I doubt that it's an intentional penalty - thinking as a programmer myself, it might be the case that, since keyword weighting was introduced, one factor in the best match could be the average keyword weight, which would improve the position for images with keywords that work well.
If that were the case, then new keywords would have zero weight and would reduce the average, at least temporarily, until they were actually used to buy an image, so additional keywords would be bad for the best match.
On the other hand, a buyer has to find the image before they can buy it - so don't let that discourage you from adding appropriate keywords!
Just speculation though, I have no special knowledge of the best match...