When Br2 adds to an alkene with different substituents on each of the two sp2 carbons, such

Question:

When Br2 adds to an alkene with different substituents on each of the two sp2 carbons, such as cis 2-heptene, identical amounts of the two threo enantiomers are obtained even though Br- is more likely to attack the less sterically hindered carbon atom of the bromonium ion. Explain why identical amounts of the stereoisomers are obtained.