Unlike the Rashi chart, which assumes every house is a fixed 30-degree sign (e.g., Aries is always the 1st house if the Ascendant is in Aries), the Bhava Chalit chart calculates house boundaries based on the of your Ascendant.
In the most common "Equal House" system, the calculator adds and subtracts 15 degrees from this midpoint to find the start and end points (Sandhis) of the house.
Different calculators may use varying mathematical models, leading to slightly different results:
The calculator takes your Ascendant (Lagna) degree and sets it as the "midpoint" of the first house.