That's just not true (at least in terms of the coronavirus). Ferguson presented a couple of different models in his original paper but for some reason everyone only mentions the one that had the 550k figure. But in actual fact, that was only:
In the very same paper, there was a different model, where with a:
As I say, only the 550k model seems to get mentioned and the 5,600 model is totally ignored by everybody.
Later on of course, he published another model based on a revised reproductive number (R0) of 3 and allowing for the Government's social distancing policies, which reflects the fact that:
So unsurprisingly, three different models based on three different sets of assumptions, gave three different potential outcomes. Ferguson didn't say any one of them was "the truth" and inevitable, only that this is what the numbers could be, if certain things happened.
Neil Ferguson projections of CoVid-19 deaths