But NYT article where the quote originated is from December 2020 (ie months before the Delta variant was officially named and more than half a year before it hi

But NYT article where the quote originated is from December 2020 (ie months before the Delta variant was officially named and more than half a year before it hit the US), so I don't think that is an explanation for why the number changed. I think the most likely explanation is that public health officials believed that citing a 60-70 number would feel more achievable, and thus encourage people to continue masking/distancing until a vaccine was available. If they has said 85% in May 2020, maybe people would have thought it was hopeless and just opened up immediately. Whether they judged correctly or not, I don't think it was a good idea to bend the truth because it erodes trust in institutions.