vaughn writes:

For example: if your doctor diagnoses you with type 2 diabetes, you
will be required to report that on your medical application and then you will be
required to jump through the appropriate hoops to get your medical, even though
you may (probably will) pass the AME's very crude urine-based blood sugar test.

The doctor doesn't have to diagnose it. He may simply have a concern about
high blood glucose, which is not the same as a diagnosis of diabetes. The
diagnosis must be reported, but not just the doctor's verbally articulated
concern. And indeed, an occasionally high blood glucose doesn't necessarily
equate to diabetes, so jumping to a diagnosis isn't always warranted.

In contrast, if there really is diabetes, then it's better to get it diagnosed
and fixed, and whether this occurs before or after the FAA medical is
irrelevant. If you are diagnosed a day after the FAA medical, then you are no
longer fit to fly until you get it fixed.