On topic reply to an off topic comment. :) Could it also be because people use "don't know" and "don't understand" to mean the same thing in many contexts? With "don't understand" being longer to type and speak and also being more "formal" in communication than "don't know", perhaps "don't know" is used as a substitute?

In the Bush example, someone saying "I don't know why anyone voted for Bush" probably means "I don't really understand why (and with what thought process) anyone voted for Bush after everything we know about the matter."