Have you seen one app place Venus in house 5 and another in house 6? First check that both charts use the same birth details. Then look for a setting called the house system.
Houses can be divided in different ways
Western astrologers use several methods to draw the twelve houses. Whole Sign and Placidus are two examples. A planet near a house boundary may appear in a different numbered house when you switch methods.
Compare the same birth details
Check the birth date, exact time, and place in both apps. These details affect the houses. If they match, compare each app's house setting. Changing only that setting can change a house label without changing the planet's zodiac sign.
Pick one setting while you learn
Astrologers do not all agree on one best house method. For a clear first reading, keep one method and your birth details the same. You can compare methods later, knowing which setting changed.
A few common questions
Did the planet move when its house number changed?
No. Changing only the house method redraws house boundaries; it does not move the planet's zodiac position.
Sources & further reading
Astrology sources describe their own traditions and methods; they do not establish predictive accuracy. The writing exercises are original Lodestar editorial material.