there's not much difference. "kau" should be pronounced like "cow" (cf. Bauer/Bower). The name seems to have originally been recorded as Küml or Kümmel in the 18th C. (probably by folk-etymology of a Czech name), subsequently known as Koukl, with the Kaukl spelling dominant among his grandchildren (still in Bohemia). The change may be purely administrative, as in the 18th C German was the dominant language of administration in Bohemia.