Because of the downwash created by the flow over the wings, a higher streamline hits the tips than hits the roots - the flow being deflected downwards along the chord, which for a swept wing means also along the span. Thus, to the airflow, a flying swept wing, particularly at high AoA appears to have dihedral.
Swept wings have much less lateral stability at low AoA, because there's less downwash being created - at the zero-lift AoA it should have none.
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