Epoxidation with peroxyacids In this particular epoxidation/hydroxylation reaction, why does the epoxide only form at one of the double bonds and not both? 

The compound initially reacts with the peroxyacid to form an epoxide, follow by an acid catalyzed epoxide opening resulting in syn-stereochemistry. My initial guess was that the epoxide would only form by the double bond that is not sterically hindered. I don't understand why the epoxide only forms at the double bond flanked by two methyl groups and not both.
This source states that MCPBA gives preferential epoxidation of the more substituted alkene. This source explains that due to intra-molecular hydrogen bonding in solution, the high degree of polarisation results in an electrophilic oxygen atom that adds to alkenes. The most substituted alkene will be the most electron-rich alkene and hence the most nucleophilic. This is supported by the data shown here and here 4 on the relative rate of reaction of unsubstituted/mono/di/tri/tetra subsituted alkenes.