That is because a “smarter” similar keywords algorithm is applied to the taxonomy term sources, and not the post title sources. That is because of performance reasons. While the keyword “tst” is only one character away from “test” it is extremely difficult to gather and compute the suggestions without making a huge impact on the server perfromance.
Please mind, the plugin tries as best as possible to match suggestions, but of course the google keyword suggestions source is far superior – at it comes directly from google, and it does not have to consider performance related issues.