There is a problem in Scottish spelling when it comes to English. In fact the same problem exists in the Greek, Cyrilic, and Semitic alphabets when it comes to this particular sound. The word "hooper" is simply, another way of idenifying a "cooper", one who puts hoops on tuns.
Similar problems exist between the Greek word that gives us choir, chorus, and choreography and the Roumanian, Bulgarian, and Hebrew words related to dance, "hor(a/o)".
Also shows up with strangers in Scotland who confuse loch with lock, and people who do not know how to pronounce J.S.´s, J.C.´s, and P.D.Q´s, last name.