No, it is an artificat of designing for a high sucess probability. For a simple example, let's say you want a robot for a 1 year mission with 90% probability of it lasting two years. You design the robot so that it has a failure rate of 5% per year. As a result it is highly likely (~80%) that it will last four years or twice the required mission length.

Obviously this is a very simplified example, but hopefully it illustrates the point that you cannot easily determine the exactly lifespan of a piece of equipment.