This situation may arise for a variety of reasons. Favoritism might be one such reason, but you should look at the issue in a positive manner. It may arise from a change in policy, as many have suggested. In such a case, your company may also have plans to compensate you with a promoted position and salary/benefits in the near future, possibly after your next appraisal, to bring parity. You may also look at both of your academic backgrounds and achievements. That might give you an indication for such a corporate decision.
I suggest you be patient and find out if they have any plans to bring parity between you two. We, as humans, have a tendency to compare ourselves with people who are above us rather than those who are below us. With careful observation, you may find several employees in your organization with similar credentials as you but are recruited at a lower level than you. Evident to basic human characteristics, you have never felt bothered by such incidents.
Regards,
Tareq Hamid
Manager - HR
Bank Alfalah Limited
Bangladesh Operations