Topic Volume is the monthly average estimated search volume for a particular phrase or keyword. Just like any other software, our search volume figures are estimates and not absolute values. We use Google-based data for our search volume estimates. Other software may include clickstream data in their calculations. This isn’t necessarily the most reliable approach as it’s collected through a variety of plugins, extensions, and browsers.
Read 7 Reasons Monthly Keyword Search Volume Can Be a Poor Metric for more details about the challenges in estimating search volume and relying on those estimates.