There are several theories which have tried to explain his contradictory attitudes toward Hideyoshi: from the grudge he started to develop due to the loss of his adopted son, Tadachika SHIMAZU, and from the reasoning that he tried to shoulder all the displeasure that the Shimazu family received, etc, however, there is no proven theory and this contradiction has come to be one of the biggest mysteries about Toshihisa in later years.
この征伐前後における彼の矛盾した秀吉への対応には、秀吉の九州征伐で婿養子・島津忠隣を失ったことからの怨恨説、今征伐における秀吉の島津家への不興を一身に背負おうとした説など複数あるが、定説はなく歳久の後年を語る最大の謎である。