While various Analytics softwares help you to track & get basic traffic and visitor data, they also have few yet hazardously significant holes. There are many reasons for getting irrelevant, non required and Null/missing data. Some examples of the areas from which you might be getting conflicting data are Internal Traffic, Direct Traffic, Mobile Traffic, Private Browsing Traffic, Keyword Traffic, Social Media etc.
To know more, visit the following link:
http://blog.kissmetrics.com/what-analytics-is-hiding/