Why would a doctor perform an oophorectomy when assured that only a cystectomy?

Difficult cyst: Sometimes the findings at surgery are more complicated that what was expected. The doctor may have encountered dense adhesions so that tissue planes were distorted and removal of the ovary may have been safer than removal of the cyst. The cyst may have replaced the ovary and been stuck to it so the only way to fully remove the cyst was by removing the ovary. Or perhaps cancer was suspected.