Well, without telling it your weight and the difficulty of your walk i cant see how its result can be perfect. Its probably close enough though, is the average for a person not 10,000 steps a day?
them things arent very accurate, if your sitting down and move slightly, it says you've took a step, am i right?
You are used to burning that many calories though, so don't think it knocks 400 off what you are eating, your body is accustomed to it, you'll have to up your walking and take off the extra for it to have an effect.
You burn calories just by living.. otherwise we'd all be MASSIVELY overweight.. yes, 400kcals would be about right for 18,000 steps, possibly even lower than expected - it's probably set to underestimate in case you sue when you don't lose weight or something? i'd say as a general rule, -300kcals per 10,000 steps, plus the extra calories burnt in every day.. living!