can't get the same number of msg when play the same rosbag

edit

I play the rosbag as -r 10 speed and use a node to subscribe to the topic.

But sometimes I can't subscribe to the same number of msg.

Maybe I play 10 times the rosbag, and then can get 5-6 times with a same number, others are different.

My rosbag is recorded as 1000Hz. I will write down this all msg into a .txt file, so actually the number of them is important for me.

That is unstable! BUT WHY????