Only a guess but different observers & telescopes & theories at the time may have postulated this or that occuring at different times. Will we all die when the tail first crosses Earth or will it take a while?
The comet was actually nearest Earth on May 20 (as I have it.)

Even in the 1986 event, many diffedrent dates were given for when it would do this or that.

Thank you for your additional contribution (of very, very many) to the Happy File. The usual recompense is herewith dispatched.