Have a close look at the graph and there does appear to be a series of steps ... As one file finishes there is less overhead both traffic and processing so the Ping response can occur fractionally faster.
And from experience, I have seen four big downloads started a few seconds apart. With one it get 100%, with two maybe a 60:40 split, with three 45:35:20 and four 40:25:20:15 or similar - they often seem to run at different speeds - which could explain the steps and one finishes ...
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