Besides preventive measures having an effect on the actual numbers, it does seem quite possible that the lack of test kits had a ballooning effect on the predicted numbers.
For example- I hear coronavirus is roughly 2x as contagious as the regular flu. So if the regular flu typically infects 5-20% of America every year, it doesn't seem terribly far-fetched to think that coronavirus could infect 30% .. that's roughly 98,400,000 people. The death rate seems to be roughly 1%, which would mean nearly a million deaths..
However...
when there was less test kits, only older people with the worst symptoms were tested..(and these people were also more likely to die)..which would skew the death rate. So if the death rate was originally 2-3% this could have pushed the death forecast into the 2-3 million range. I'm not sure if this is what happened, I'm just saying it seems possible.