Why you should upload a GEDCOM with
Person number 1 exports her complete GEDCOM shown in blue. Her known genealogy goes back to her Grandmother shown as number 3, but not to number 3's parents (shown in green).
Person number 2 exports a GEDCOM containing only her direct ancestors which doesn't include siblings of ancestors, or the descendants of siblings. It includes the part shown in red and green, but not the female shown as number 3 (sister of one of number 2's Grandfathers), even though she has that information in her complete genealogy.
Because the two GEDCOMs being compared do not both contain the common ancestor "3", no relationship can be established.
If person number 2 had exported her entire known genealogy, a comparison of both GEDCOMs would have indicated a match.
It's best to upload a GEDCOM containing your entire known genealogy. If you're worried about privacy of living relatives, you can "privatize" before exporting. Don't worry about including 'too much' information. It's much better than not including enough information!