There are different tests available, with different methods.Normal blood picture vs inflammation makers, liver enzymes, blood count, etc. Same with stool samples. Some are just put under the microscope to look at, some are grown to check on live bacteria, fungus, parasites...you can also measure inflammation, blood, the grade of digestion. And even DNA of what is living inside of you. They just want to make sure they didnt forget anything, or overlook anything. Not all tests are done at the same time, since some of them are expensive. No need for a whole bloodpicture if a cellcount would do, right?
And then there is the fact that things can change fast. A negative parasite test today doesn't mean it is not positive tomorrow.
And maybe the MD used a different lab than the gastro. They dont necessarily trust each other.
They just want to make sure that they didnt forget anything.