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.
When you’re running the Rank Checker feature sometimes you see Google’s captchas pop up and which can be annoying. I have only experienced this on a computer with an old OS (Windows XP) though; I haven’t seen this on Windows 7 or Windows 10 machines. Again, if you close the software and reopen it after a few minutes, everything will be okay again. UPDATE: one of the latest updates fixed this issue.
Keyword Overview looks at the entered term on its own and addresses expected searches per month, the difficulty of competing against Page 1 results for the same term, a rough estimate of organic click-through opportunities (versus paid ads and non-organic positions on the search engine results page), and the perceived priority that the user should assign to optimization for this term, where the higher the score, the higher the demand and a lowered level of competition.
Basically, Google shows the autocomplete suggestions whenever you start typing anything into Google search box. It is in Google's best interest to show the most relevant keywords in the autocomplete suggestions. Keywords that would help Google to retrieve the most relevant websites and help users find the most relevant content for their search query.

Recently I had a dilemma with one of my projects, it is related to ecards and many people still using word “cards” instead of “ecards” but Google Keyword Planner and some other tools showed almost the same information for both keywords. At the same time I did not want to have many words “cards” and “ecards” on the landing pages. Semrush helped very much. I found correct data and made a nice PPC campaign.

Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website.  In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC).  I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.