[en] Recent investigations of stretched M 6 transitions in the s-d shell have uncovered the unexpected systematic trend that isoscalar M 6 transitions are two to three times more retarded than the isovector ones. We present the result of a microscopic calculation analyzing the different mechanisms usually invoked in the formation of the quenching and suggest that an additional argument based on the difference in structure between isoscalar and isovector transition densities may be playing a predominant role in explaining the data. (orig.)