When an alkane with different types of C-H bonds, such

When an alkane with different types of C-H bonds, such as propane, reacts with an equimolar mixture of Br2 and Cl2, the selectivity in the formation of the brominated products is much worse than that observed when reaction is carried out with Br2 alone. (In fact, it is very similar to the selectivity for chlorination.) Explain.