I think you are making a mistake here comparing probability of death in a general population and a population of migrant workers. In general population there are people of different ages, including very old people, people with incurable illnesses, people with poor health. For comparison, migrants are generally more healthy because otherwise they wouldn't be able to do their job. For example, migrant workers are unlikely to die from being too old. Therefore, the probability of death from natural reasons for a migrant should be lower.

So we can assume that those 6500 people probably died because of other reasons, like traffic accidents, workplace accidents, bad living conditions, not getting medical treatment, and so on.