A confident guess becomes harder to correct after others have built plans around it.

An estimate offered quickly gets treated as a figure, and once other work is scheduled against it, correcting it costs more than making it did. People stop asking where it came from. Whoever guessed may know perfectly well how soft the number was, and by then saying so means disrupting several people who have already moved. Certainty is cheap at the moment of speaking and expensive afterwards.

When you give a number you are unsure of, say the range and the basis in the same breath. And if a figure you offered has hardened, correct it early, while the correction is still small.