It's fairly accurate. There's no exact science to this but the vast majority of crawlers won't influence the number. The logic used is very similar to tracking actual visits to the site for participation-based badges.
And when I say vast majority of crawlers, I'm leaving room for academic settings where a crawler fetches a set of links, and then humans go through them and create metadata based on what they see, or other such settings where we end up firing a visit for non-typical behavior.
If anything, the reached indicator is probably more on the conservative side than anything. You can go through 5 or 6 results in the top 10 of searches in way under the amount of time that would actually register (depending on how aggressively you close tabs).
The effect does tend to snowball as people find pages with posts you wrote and spend a little time on them, because that's a key metric for two things:
- We register the visit
- Google notices that people spend time on that particular page, and gives it more weight
So, while the number of people you've reached is a rough aggregate estimate, some contributions you've written can disproportionately influence that number and that's .. essentially what we'd expect. Some of my best answers are on posts that kind of languish in obscurity and there's just no great way to solve for that in such a generic indicator.
But overall, yes - it's a number that's probably way more understated than overstated, and something you should feel pretty good about. Jay Hanlon and Oded Coster invested a huge amount of time in it just so folks got a better sense of their net impact on people's day-to-day lives, so, enjoy it :)