Making up numbers doesn't make things better for you or for delivery of the project. But it makes you liable to the boss, so they can make your head roll if they need to.

A surprising number of people in business think this way: not that they need to make sure of something, but just that they need someone to tell them everything will be OK, and then put the blame on if anyone is unhappy. That someone will naturally be whoever is at the bottom of the totem pole.