Why might Cosby's actual jail time be zero?

Because Cosby is wealthy enough to appeal every ruling all the way up to the Supreme Court if necessary. So, it is possible Bill Cosby will actually die before serving any jail time. Also, Cosby is now legally blind so this is a factor too. He wouldn't survive in most jails because of his blindness. So, sentencing him would potentially be a death sentence within a week or two because of this.