You're counting the entire US military budget as "NATO spending" which is not a useful way to look at things. We only contribute roughly $800M to NATO's shared budget (about 16%)[1]
Also, from what I recall, the reason why US military budget appears so high is also because pensions for retired personnel (the highest invoice) are included in the military budget, whereas they are not e.g. in Italy where it's a different budget.
Also, from what I recall, the reason why US military budget appears so high is also because pensions for retired personnel (the highest invoice) are included in the military budget, whereas they are not e.g. in Italy where it's a different budget.