A few things to note with the number of returned pages reported by Google. First, it is only an estimate, and is very rarely within 10% of the actual number of matching pages. In fact, if you wait a few minutes and do the search again the number can change by several percentage points. This is even the case when searching with the "site:" parameter on a large site. Second, Google will filter out pages that seem to be similar in content to one another, as well as filtering out pages that have limited content. This can cause the number to vary from the number reported by other search engines.

As an example, in a search for "air conditioner rental", Google says there are 30,600 results. However, only 391 pages actually show up in the SERPs. Subsequent searches only returned 345 and 215 documents, respectively (could not get the filter disabled at all, maybe a bug?).

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