The difference between the tests on different days (if you didn't do them with various other downloads happening) would mean there's something else which is slowing the line down on that day. E.g. more people connected taday than yesterday means the main line shared between yourself and them is more congested. To use the water pipe analogy again: it would be as if your neighbour is filling his pool and you find that your shower head is now only dribbling.
Another thing that might be causing it is some restriction on the line, i.e. something went wrong somewhere. Again using the pipe - it's as if the pipe sprang a leak somewhere, or the plumbers are working on it and turned it off or rerouted your house (even temporarily) to use another pipe while they're doing something to the usual pipe.
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