There's no secret messaging or sign that can tell you, a company is about to spend with you. The interested party will say whether they want to buy or not.
I can understand why some don't want to divulge traffic stats, but other things like demographics are usually available on websites, and so isnt a problem. Looking at how long a website/biz has been established is also important, as have seen a site with claims of 1 million hits a month, yet it had only been about for 12 months...
So yeah, some will just lie and hope this gets them the business. When I search for ad space, I do expect some bravado/bs about stats as it shows a hunger for the business, but too much bravado is a turn off.
A good rule is not to pay too much attention to stats, but use it as a rough guide to the website/biz. Personally I'd check to see if the site has a specific user type/following... Their a safer bet when buying ads.