It's because we like to have very ambitious defence policies, and the military likes to have defence policies that tell it to buy the equipment it needs. As a result, politicians, the Department of Finance and the armed forces all have an interest in ensuring that costs are very low, in order to have policies that allow them to buy equipment. However, at the end of the day, you're dealing with the real cost in the long run.
In short, everyone has their reason to be optimistic about costs.