Vedic and Chaldean numerology assign each digit 1 through 9 to one of the nine grahas — the seven classical planets plus two shadow points, Rahu and Ketu, which mark where the Moon's orbit intersects the ecliptic. 7 draws Ketu, the second of those shadow points.
Where Rahu (paired with 4) is read as worldly ambition and disruption, Ketu is read almost as its mirror image: detachment from the material world, spiritual introspection, and a pull toward the unseen. That reading maps unusually well onto numerology's own independent description of 7 as introspective and drawn to deeper truths — the two traditions reinforcing each other rather than sitting in tension, as they do for 4.
Western sources that don't use the Vedic shadow-point framework often substitute Neptune for Ketu, since Neptune carries a comparable astrological reputation for mysticism, intuition, and the dissolving of clear boundaries — a recognizable stand-in for readers unfamiliar with Vedic astrology's specific vocabulary.