anchor: It's more credible because it is more limited and because it's more plausible that you'll do something if it's something you have a good reason to do, whether you've said you'll do it or not.
positive: Now that you have a good reason to do something, you're more likely to do it even if you didn't intend to.
negative: It's less credible now because you won't do something as there are no good reasons.