Sorry, I don’t know. Could be the cells if they had aged a lot but could also be another setting? I agree with @ShutterShock, try the battery builders thread as it has a lot more participants who can help out.
Are you sure it’s stopping before 4.15V? Could it be charging to 4.15V and just settling back down to 4.09V? That could indicate aging cells or a high-resistance connection, or connections, somewhere.