It's slightly misleading to say that because other countries, like China and Russia, aren't as transparent with their budgets and there is some hiding of military spending in other parts of their government, from what I've read.
It’s hard to imagine we secretly spend significantly more than we say we do, because we publicly spend a lot. I’m sure there are some secret projects with secret funding, but they couldn’t be more than a rounding error compared to the amount of money the military officially spends.