That happened with my father, too. Several times we were told, quite solemnly "He won't make it through the night", and then at 7:00 a.m. he'd call my mom asking when we were coming over to see him.
We just cannot discount the strength of the will to live.
Unfortunately the cancer finally took Dad at the age of 52 in 1978 about 9 months after he was diagnosed.
I am now 7 years older than he was when he died. That's a very sobering thought.