"created_at:" timestamp in JSON tweet

[estathop]

I noticed in this API documentation (https://developer.twitter.com/en/docs/tweets/data-dictionary/overview/intro-to-tweet-json) that field "created_at" of the tweet JSON is supposed to be the timestamp when the relevant tweet was created and published on twitter.
On the contrary from trial and error I realized that in the large text files every "created_at" field indicates a timestamp when the tweet was crawled and not created.
Is this an erroneous behavioral or is this how it is supposed to function ? Or whether you choose
"item_id" : 1 or latter it only fetches the latest 10 days for the standard API, ( the free one )