A Google News Help thread has a complaint that the number shown in the "sources" section of Google News clusters is wrong.
Here is an example, this says 3 but it leads to 4 sources:


I honestly never thought the number was exact. I though it was an estimate, like the number of search results shown in Google. But a Google representative replied like it is a bug.
Google's Archi Sarkar-Goyal said:
Thanks Lmobi for reporting this. We are working on resolving on this issue soon.
Makes you wonder...
Forum discussion at Google News Help.

Comments:
Aaron Bradley
01/10/2013 01:03 am
In your example the source count was only off by one number. That's pretty fabulous, as I've never clicked the "all X sources" and got anything *remotely* resembling X. Glad to hear Google is working on this.