[yeukhon]

Nice study. I think it's quite intuitive. When you are forced to spell you are force to say something. So by saying "I don't know" it is natural to continue with "I don't know but I think" and then it continues with more words.

Whereas to tell someone in advance "don't you dare to tell me I don't know" the person will just hold on to the thoughts until he or she has to spell that thought out (because he or she has to say something to get out of the misery).