In the Western occult tradition, each planet can be represented by a seal or diagram. The seal is based off of the planet's magic square, with the seal theoretically touching every number within the square, although in practice that is not always apparently the case.
The Seal of Mars does not follow the convention of overlapping every number of the magic square of Mars. Three squares are missed entirely: 1, 5, and 21.
The Mars seal is structurally similar to the seal of Venus. In mythology, Mars and Venus are lovers and thus are a pairing. In an earth-centric cosmology (such as what occultists worked within when these seals were designed), Mars and Venus are the planets closest to the Sun, which holds a special position and role within the cosmology.
Read more: Structure of the Celestial Realm, and the Importance of the Sun
The specifics of why the seals of Mars and Venus are constructed as they are is much more elusive than for the other seals.