Why does will smith's I am a Legend have two different endings?

Original ending: will smith gave her blood to her mother and son in the last laboratory, but she died with the zombies. The mother and daughter embarked on the road of saving mankind with antibody serum.

In addition, the ending: will smith suddenly changed tactics when resisting the zombie attack, and pushed the female zombie out of the glass door. Other zombies stopped when they saw their companions. At this time, one of them hugged the female zombie, and the female zombie miraculously woke up at this time, as if seeing the hope of removing the virus again, so the zombies left one after another, and will smith followed the mother and daughter on the road to saving mankind.