Just to reiterate Edwin Ashworth's explanation in a comment above, I think that the sentence "People of all races incorrectly estimated X" is much less likely to mean "All people of every race incorrectly estimated X" than it is to mean "Some people of every race incorrectly estimated X."
In part this conclusion reflects probability: Intuitively, it seems far more likely that at least one European, at least one Asian, at least one African, at least one Native American, and at least one aboriginal Australian might incorrectly estimate X, than that every single European, Asian, African, Native American, and aboriginal Australian might do so. The exception to this normal order of expectation would be if the estimate involved an extremely esoteric question, such as How many stars are in the Crab Nebula?—in which case the fact that no one, regardless of race, estimated the number correctly within a precision of ±1000 stars is unlikely to be deemed newsworthy.
But in part this conclusion also reflects how a speaker or writer is likely to frame a piece of extraordinary information. If not one person interviewed happened to estimate X correctly, it seems to me that the reporter would be more likely to express this fact in the form "Not one person, regardless of race, correctly estimated X." Not only is the wording more dramatic than the rather flat-sounding "People of all races incorrectly estimated X," but it is not susceptible to misinterpretation, as the latter obviously is.
In view of the magnitude of the dropped word's significance to the sense of the underlying idea, it is far more likely that "some" would drop out of the statement "Some people of all races incorrectly estimated X" than that "all" would drop out of the statement "All people of all races incorrectly estimated X." The same thing happens in "Tears of a Clown": "People say I'm the life of the party, 'cause I tell a joke or two," sings Smokey Robinson. But it would be a stretch to conclude that he means "All people say I'm the life of the party," and not "Some people say I'm the life of the party."