It is because the Center for Disease Control inaccurately reports effectiveness based on outdated, low quality research and everyone, medical professionals and the general public,  uses the CDC website as the go to source for up to date, accurate effectiveness data!

“Based on the most up-to-date and highest quality published medical research, the effectiveness rates of Fertility Awareness Based Methods (FABMs) with correct use are between 95 and 99.5%, depending on the method ii. Even with typical use, the effectiveness rates of FABMs are comparable to most commonly used forms of birth control.
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Please take a few minutes to read the link below and sign the petition to ask the CDC to report accurate data on side effect free, effective Fertility Awareness-Based Methods.