rosbag play cannot return back?

Hello, I use `rosbag play -s 10 -u 10 a.bag` to publish the recorded messages. But I find that when I use the command before, I cannot use `rosbag play -s 1 -u 1 a.bag` to get the right result, which means if I use the parameter `-s` to set a particular time slot, I cannot play back to a time slot before. I want to ask if it is the limitation of `rosbag`, or if I use it in the wrong way. Thank you.