Why are some birth control pills a weaker dose? what exactly does that mean?

Estrogen: The estrogen content, specifically, the ethanol estradiol component in a bcp is typically referred to as the "strength" of a pill. This can be 35 micrograms or 0.035 milligrams at the higher end, or 20 micrograms (0.020 mg) at the lower end. Pills with low dose estrogen need to be taken exactly the same time, or else the likelihood of breakthrough bleeding increases.