Tropical and sidereal zodiacs
Western astrology commonly uses the tropical zodiac, which is anchored to the equinoxes and seasons. Vedic astrology commonly uses a sidereal zodiac, which accounts for precession through an ayanamsha.
Why signs can change
The offset between tropical and sidereal zodiacs is large enough that many planets move into the preceding sign in a sidereal chart. This is a difference in reference system, not necessarily a calculation error.
Interpret the system as a whole
Vedic and Western astrology have distinct techniques, emphases, and interpretive lineages. Mixing a sidereal placement with a tropical interpretation can create confusion.
Which should you choose?
Use the system you want to study consistently. Exploring both can be meaningful when each is labeled clearly and interpreted on its own terms.