The most common reason for getting lots of inconsistent answers from the same contacts answering your surveys is that respondees can answer the survey multiple times. This can of course be useful in many cases, but in some cases it can skew the survey results.
To combat this, you can set the survey to allow only one response per respondee. To do this, start by navigating to Surveys on the blue navigation bar. Next, select the survey you want to edit.
Once you're in the survey editor, navigate to Settings on the navigation bar on the left. Here, scroll down to the Responses section and check the box on the Allow single response for responder. Now the survey only accepts one answer from a device.