I believe this is when the API call happens soon after the user removed a tweet?
This is also happening to me when hitting the Favorites/list endpoint.
See: Favorites/list endpoint not returning correct # of tweets
Are you excluding retweets or has the user muted anyone? As far as I remember some things, like excluding retweets and filtering muted tweets are done after the count is applied, so there are some situations where it is possible that not the full
count number of tweets is returned.
I think it’ll even do it if a tweet was deleted.
Specifies the number of tweets to try and retrieve, up to a maximum of 200 per distinct request. The value of count is best thought of as a limit to the number of tweets to return because suspended or deleted content is removed after the count has been applied.