In order to understand this strange phenomenon, we have to consider both CuO and Cu2O.Both are nonstoichiometric oxides and both show CTTM phenomenon where the electronic charge from the filled MOs lying mainly on Oxygen is transferred to the vacant Mos lying mainly on copper ion.Being nonstoichiometric means that copper and oxygen do not obey simple valency rule. The charge on copper in cupric oxide is +1.4[not+2] while charge on copper in cuprous oxide is +0.9[not+1].The CTTM effect in cupric oxide being much stronger will reduce the charge of its copper from +1.4 to +0.8 while charge on copper of cuprous oxide will be reduced from +0.9 to +0.7.
In a way, the copper ion in cupric oxide will be analogous to Cu (+1) and thus is