Rabbi Steinsaltz writes:
Humility is not falsehood or self-deception.
A tall person who says that he is short is not being humble; he is not telling the truth.
By the same token, a righteous man who says that he is a sinner is just as untruthful as is a sinner who proclaims himself righteous.
The humble person might underestimate himself but only regarding matters that are subjective and nebulous, not empirical facts.
True humility is a person’s acknowledgment of his insufficiency before God or his insufficiency before his own potential.
Such humility can be found only among the truly great-those who are aware of the loftiness of their own soul, who know the true extent of their capacity, and who understand how distant is the actual from the potential.
Such subjective self-judgment is the source of true humility.
Humility has no place, however, regarding things that are measurable by objective criteria.
–Rabbi Adin Steinsaltz