Tropical and sidereal

The tropical zodiac is anchored to the seasons: 0° Aries is where the Sun stands at the March equinox. The sidereal zodiac is anchored to the stars: it is the tropical ring shifted back by the ayanamsa. The ayanamsa itself is a choice, since several definitions exist, and Genastro uses the Lahiri one.

The two start aligned and drift apart, because Earth's axis wobbles in a cycle of about 26,000 years. That precession slides the equinox backward against the stars. Western astrology mostly uses the tropical zodiac. Vedic astrology uses the sidereal one, usually with Whole Sign houses, so its Ascendant and its houses shift as well.

Switch zodiacs. The planets do not move. The ring underneath them rotates by the ayanamsa, and each body picks up the sign that is now beneath it.

A sign is always a 30° slice, in either zodiac. The constellations the signs are named after vary in width, and the Sun crosses thirteen of them, not twelve. So "the Sun is in Pisces" means a slice of the ring, not that the Sun is in front of the stars of Pisces.

The two zodiacs measure different things, and neither is wrong. The frequency figures on this site were simulated in the tropical zodiac. They describe tropical placements only, and Genastro says so whenever the sidereal switch is on. In a sidereal chart the ring is shifted, so a share measured for the tropical sign does not carry over to the sidereal one.