The Torah discusses two different types of uterine bleeding: dam niddah and dam zivah. You are correct that, according to biblical law, a niddah may immerse on the night following the seventh day of bleeding, while in many cases a zavah must count seven clean days before immersion.

However, it is not halachically straightforward to distinguish between dam niddah and dam zivah. They are rendered distinct from each other based on the time of month bleeding begins with respect to the last menstrual cycle. There are other complications in making the Torah's distinction, such as a rabbinic dispute as to how to calculate niddah versus zivah days.

Additionally, according to biblical law, only five specific hues (four red, one black) make a woman niddah. The tradition enabling us to identify these hues had already been lost by talmudic times. Should a woman start counting the biblical seven niddah days with bleeding of a color that does not render her biblically niddah – and on a later date experience bleeding that does – she would immerse too early and inadvertently transgress the biblical injunction of niddah. By treating all uterine bleeding as potentially either niddah or zivahblood, we eliminate the chance of such a transgression.

For centuries, the response to this case of halachic doubt (with respect to colors and calendar keeping) has been to treat all uterine bleeding as possibly niddah or possibly zivah blood, and to require seven clean days under all circumstances. This approach has been codified into Jewish law.