There are a growing number of people noticing the strange results from the site: operator. Bottom line for me, this report cannot be trusted any longer - most especially not the number. Just because a url is not in the site: operator result doesn't mean it isn't indexed and getting search traffic.

The site: operator for me is pretty accurate compared to the amount of pages shown indexed in Google Webmasters. If I search for a page in one of the data centers indexes, in the normal google, I dont get any results, so the two are not reporting incorrectly in that respect. Just showing different amounts of pages overall.

I'd agree with tedster on this but would add an additional observation(s)...

Anyhow, while I found "site:" to be maybe 65% reliable (I'll expand on this) I still find the correlation (between the 'site' number and SERP traffic) to be fairly significant. The issues with 'site' I have noticed so far is
a) 'Under-reporting' - e.g., on a 100 page site, the site operator returns 10 pages, but google sends traffic to all 100.
b) There is a fluke that occurs sporadicaly (let's say once in 10 days) when the operator returns rediculous numbers, e.g., on a 100 page site I get 750 indexed pages. I have several sites and this affects let's say one in five every time it occurs.

Cheers,

VK

I've seen this before: when a significant number of pages were in the 'Supplemental Index' and some datacentres factored those in, whilst others did not. This was quite a few years ago, and the way these other indexes work is now very likely to be quite different to how things were back then. Additionally, I've seen variuous types of Duplicate Content being filtered in multiple different ways on different datacentres, leading to different counts too.