What exactly made Boromir "un-Numenorean" compared to Denethor/Faramir?

Gandalf says this to Pippin:

He is not as other men of this time, Pippin, and whatever be his
descent from father to son, by some chance the blood of Westernesse
runs nearly true in him; as it does in his other son, Faramir, and yet
did not in Boromir whom he loved best.

Why does Gandalf say so?