The treatment of initial palatals + w is quite different from that of the velars + w. For the closer contact of w with the palatals produced an early (pre-Greek) regressive assimilation of k̂w-, ĝw-, ĝhw -(through kҍ, kp ; gҍ, gb ; ghҍ, ghbh to (π)π-, (β)β-, (π)π-. For this assimilation compare Iran. sp from k̂w, zb from ĝw, ĝhw. Here no further assimilation took place as the palatals had become sibilants.
A Wed, study studied this question.