Oil Rubbed Bronze (US10B) is a "living Finish" and will change color the more it is touched. An Oil Rubbed Bronze lock that is used heavily will slowly loose its dark finish where ever it is touched and will begin to develop a bronze finish over time. In addition the oil from you hands will eventually create darker areas and the lock finish will develop a very "antique" look to it. If the lock is rarely used then this process will take longer so some locks in your home will look different than others in just a few months. You can speed up this process with some machine oil and fine steel wool if you like. This can also help to blend the color of areas that are not touched or on locks that are not used much
This change is not a defect in the finish but is intended to occur.
See these examples of how oil rubbed bronze changes.
This lock is 4 years old and is used a couple of times a week at the most.
This lock is also 4 years old and is used several times a day.