Why is the operator sometimes different in proactive mode than in the actual chat?

In proactive mode, a proactive message will show up after a certain period of time, inviting the contact to chat. In this mode, one operator from all associated operators in the group is displayed alternately.
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But as soon as the chat starts and the contact sends their first message, a specific operator is assigned to the contact. This may be a different operator than the one displayed in the preview, since there is no filtering based on availability, but only based on the operator group assigned to the widget.
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