According to the documentation, there is an initial image that is sent to calculate the latency of your network. After that, the JS agent will send the remaining messages subtracting the latency in order to calculate the bandwidth. Due to the latency subtraction, the DT server might have recorded the times for 30/100/300kbyte images at less than 100ms.
Thanks for your answer.
We can see the that it took 5.51/16.53/41.76 seconds respectively to download the 30/100/300 kbyte images which greatly exceeds 100ms. So it must take more than 100ms to download the 3kbyte image.