I've actually wondered this for a while and have figured out why this is happening.
Google thinks you've made a typo.
Type in "iisoo" and wow!!! 226 million searches!!
Now type in "yahoo" and.... oh, 226 million searches...
Because the word is so uncommon, Google can't fathom that you actually meant to search for that word. Unlike in Google search there's a "did you mean..." section, that doesn't exist in the keyword planner. So you're left thinking it's somehow a popular word.
But if you see a "did you mean..." in Google search, and you're seeing a high number in the keyword planner, it means you're being autocorrected to something else... in this case, "yahoo".