The meaning of the frequency value in rosbags

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Dear all,

i downloaded a rosbag from the internet and looked at the info of this file:

path:        ../raw/Vehicle1_velodyne_pos.bag
version:     2.0
duration:    36:10s (2170s)
start:       Aug 12 2015 09:22:18.17 (1439364138.17)
end:         Aug 12 2015 09:58:28.86 (1439366308.86)
size:        23.0 MB
messages:    152840
compression: none [29/29 chunks]
types:       tf/tfMessage [94810edda583a504dfda3829e70d7eec]
topics:      tf   152840 msgs @ 99.2 Hz : tf/tfMessage

As you can see, the topic __tf__ have 152840 messages and after that there is the number 99.2 Hz.

I computed:

 (end_time - start_time) / number of messages

and i got ca. 70 Hz.

What is the meaning of the number 99.2 Hz?