This happened to me. One of the horses here was dewormed and a fecal done 12 or 13 days after the deworming. According to this boarder's vet, the horse was "loaded with worms". The information came second hand from the boarder who isn't well versed in horses so I'm not sure how much he got right. He said the fecal showed the presence of "large and small something" (I'm guessing he meant strongyles). The vet he uses is not the most organized, and I'm wondering if the fecals perhaps got mixed up with someone elses.
I was stumped, so I had my vet do a fecal on all of the horses here a week later. All the fecals were negative.