Basically, then, your GS was having some concerning symptoms, so the hospital spent time, effort and money doing a lot of tests (as they should), and one came out with unexpected result. They even did the test again on the same sample, so they could find out if the first test got cocked up somehow without having to take more blood — but it came out with the same result.
So in order to be sure that they'd detected a real and persisting medical issue, they needed to take another sample and check it (and maybe take blood for another, different selection of tests, extra ones that they do if the test that your GS had an unexpected result for comes back showing a problem).
Maybe if it were a minutes-matter emergency, they might not have. Maybe if nothing could've possibly changed over time, they might not have. Maybe if follow-up tests weren't needed they might not have. Maybe if treatment for the problem was completely harmless, they might not have. Maybe if they already knew exactly what was going on, they might not have. Maybe if blood testing was always perfect, they might not have.
But testing isn't perfect, medical situations can evolve and develop, there was time to do another sample, they did seemingly need to do more tests, the situation might have changed, medical situations can be confusing and unclear, and treatment isn't harmless.
I think your grandson is worth the extra time, effort and cost involved in properly investigating the problem. Maybe you would prefer they didn't bother, but thankfully it's not up to you.