There are big holes in testing. When a lab looks at a specimen, they match it to known images (if doing microscopy), typically they look at the ova (eggs) because the ova has distinctive signs, meaning the ova of parasite 1 will look different than parasite 2. If they look at the adult stage, there are fewer distinctive signs and often times, there can be non-parasitic things that resemble a worm.
So one problem that arises is that there are quite a few species of parasites that can infect people, oftentimes, the images for comparison are not complete. This means that you could be infected with a worm that is not registered. Thankfully, most of the remedies that kill nematode 1 will also kill nematode 2.