There is a big difference.
Life expectancy is just that, an expectation, or an estimate of future deaths.
The difference is what has happened in the past and what is expected to happen in the future. Second, rock still has only been around about half a lifetime so many rock stars are still young. Because rock stars generally begin their rock careers at a young age not many have reached old ages yet. So most of those who have died can not help but have died young since the group in general is very young. The important lesson to learn here is to not trust every statistic you hear.