Two reasons: First, there is no "exact match search" using double quotes, Q2A ignores them and just searches for the words you typed.
Second, when searching, Q2A will look for questions containing any of the words, so in the case of "testing blabla bla" it will look for all questions containing "testing", all questions containing "blabla" and all containing "bla" and show them all.
It will order the question list by how relevant it thinks each one is, I assume based on how many times each of the words appear, and if a question contains all the words.