Depending on your topic / vertical and your geographic location the search engines may have vastly different search volumes. The tool can only possibly offer approximations. Exact search volumes are hard to find due to vanity searches, click bots, rank checkers, and other forms of automated traffic. Exceptionally valuable search terms may show far greater volume than they actually have due to various competitive commercial forces inflating search volumes due to automated search traffic.
It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.