Years ago, my lady and I went to a local aviation safety event. Full disclosure: the event was a former pro-bono client.
During our walk around the exhibit hall, my lady saw a friend from a previous job. Her friend still worked for the same company. That company purchased an exhibitor booth at the event. The company's goal with their exhibitor booth was to try and generate membership leads.
As the ladies caught up, there were the usual pleasantries, "how are you? what's new?" And then the interesting part. The woman working the booth said, "they said that they would have a few thousand people at this event. They're lucky if there are 500!"
It got worse when the woman working the exhibitor booth said something along the lines of, "we'll never do this again! They don't get anywhere near the attendance we were told."
As with all events, attendance is going to vary. If you have an outdoor event, you're all too familiar with wild attendance swings. Hence, the phrase "estimated attendance." That said ...
Event organizers need to have very accurate estimated attendance numbers in their sponsorship/exhibitor packages. Why?
Because numerous companies that leverage events, especially large events, are very savvy about calculating attendance ... your attendance numbers!
In particular, companies who do product sampling. Some of these companies can calculate your attendance using the temperature outside (for an outdoor event) against on how many samples they distribute!
Those companies calculating your attendance could have a massive impact on your event. If you're estimated numbers are way off, sophisticated event exhibitors will know.
It's your event and your call. But if you're going to give estimated attendance numbers in marketing, exhibitor, or sponsorship packages, make sure you're as accurate as possible. If not, companies are bound to leave you and never return. Which means you lose future bank.