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When a Salesforce SOQL statement selects a number of large fields (such as two or more custom fields of type long text) then Salesforce may return fewer records than defined in the page size in order to control the overall response payload size. The reduction in page size also occurs when dealing with base64 encoded fields (or blob fields), such as the Body. Remove these fields from the query if you want Salesforce to returns the number of rows specified on the page size.

Why number of groups crawled is different from the number groups showed in the UI?

This is because Salesforce treats users roles as a type of group, so they are stored in the same table as the other groups. To check the user roles go to Manage Users/Roles section of the setup page,

General 

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