Thank you for reaching out to Constant Contact API Developer Support.
It sounds like the account to which you are adding contacts has Confirmed Opt In turned on. While what you described is how contacts are added by default, when Confirmed Opt In is turned on contacts that sign themselves up are first placed into a pending (or "Awaiting Confirmation") status, and a confirmation email is sent to them. Once they click the link in that email confirming they wish to receive email, they will be updated to Explicit.
See here for details about Confirmed Opt In:
In the meantime, I'll bring this to our documentation team to see about getting something added to the docs about this as well.
Please let us know if you have any other questions!
David B. Tier II API Support Engineer
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