This code re-checks that the input cmdObj is in sync with the parsed readConcernArgs (i.e., that we didn't swap majority for local or snapshot somewhere along the command execution path). This is very error prone and wasteful and a better architecture would be if cmdObj was not allowed to contain any read concern arguments so that we can just append the ones passed to the function.

https://github.com/10gen/mongo/blob/e3d867d884892e7e59c50704e3eee1f81c55ccd5/src/mongo/s/transaction_router.h#L759