DTs default search is for 'Best Seller' as opposed to 'Most Relevant'. So old photos that have slowly built up sales keep getting pushed to the top (therefor sell more, stay top etc). Meanwhile your newer maybe more relevant and better quality photo is kept low down where there's less chance of selling because it hasn't sold.
Easy to see If you try searching yourself.
Why most folk on their forums who claim the site is still doing well signed up in the early 2000s where there was less competition, their photos sold so now are still top of the pile.