Job opening on a company's "career site" are a very poor predictor of the number of people that company would hire if could find suitable candidates.

In some cases, the listing is to satisfy an HR rule and the person to be hired has already been selected. In other cases, it's "maybe someone wonderful will wander in".

And then there are the hires that happen without a listing.

I'd guess that the actual uncertainty in the total is more a function of the uncertainty in the numbers than it is in the sampling.