If you ever login to see your Google Webmaster Tools search query reports you may notice that from day to day, the chart may display the same number of queries.
One person questioned this in Google Webmaster Help asking how this is possible. This webmaster didn't believe his eyes and asked, "How come that the queries can be the same 4 days in a row?"
Here is a screen shot from my report and I see similar patterns:
Google's JohnMu confirmed this reality in the Google Webmaster Help thread and said it was due to how the numbers were "bucketed" in their system. John said:
This is due to the way that the numbers are "bucketed" in our system. Basically, to simplify the UI a bit, ca. the values 160 to 260 may get reduced to 210 and shown that way in the user interface. In practice, if the numbers are the same for you, this means that it has been relatively stable for that time. We're looking into ways of making this a bit clearer or providing more exact numbers, but in the meantime if you see this, I wouldn't worry about it :).
Just another one of those deeper interesting tidbits gleamed from tough questions answered by Google.
Forum discussion at Google Webmaster Help.