Dozens of reasons. Biology rarely ever is so simple.

- Mutation/Infectiousness changes - Behavioral changes - Testing changes (more/less/precision) - Reporting Changes (like changing all reporting from CDC) - Political interference/ Wishful thinking - Non-linearity (the virus has tagged the easier victims and is now moving slower into the more distanced population) - Seasonal patterns - Any combination of the above.

Frankly, its insanity to assume herd immunity at this point in time, especially with uncertainty about length of immune protection from primary and t-cell response.

Its a virus, it writes the rules and we barely have managed to decipher some of them, this article reads like a perverted Mission Accomplished on an aircraft carrier.