What are 'News Revisions'?

This may be a ridiculous newbie question, but I've searched this forum and others, and read countless articles and still can't find a DIRECT answer to this:

When the previous number for a news item is posted on FF alongside the forecast number, why is it that right at the time of the latest news release they often alter the PREVIOUS number a few points? How can that number be altered? Isn't that a hard number from the previous news release?

And if there are other factors that enter into it, and the previous number needs to be revised, why isn't it just a few days later instead of right at the time of the next news release?

Does this question make sense? I would certainly appreciate anyone shedding a little light on this.

Thanks!

The NFP is the best example to use. The methods used to collect data on a monthly basis are dodgy, to say the least. But they need to come up with some sort of number, so they do. On a quarterly basis, they're able to contact more people and collect better data, ie actual jobs added. You probably wouldn't be far off in calling the monthly data a "guesstimate".

Over the course of a year, the data keeps coming in, and it's of better quality, read "more reliable" which is why we saw that massive year-to-year revision just recently. So, think of it this way. First number is a guess while the revisions reflect the true number as time goes on and data comes in.

Thanks Andrew, that makes sense. I guess I assumed they were working with hard data and figures, but I guess that was a little naive. Who would have guessed the governments method of gathering data was sketchy!