The Dragon Newsletter has a tip on how you can improve accuracy and reduce word recognition errors in Dragon Naturally Speaking:

When Dragon occasionally misrecognizes a word, it’s tempting to simply use the keyboard to delete the text and type in the correct word or phrase. Similarly, the “Scratch that” command makes it fast and easy to delete the transcription of the previous utterance, or string of dictated text. However, overuse of this command may keep you from achieving even higher levels of accuracy with Dragon. This is because simply deleting the text doesn’t allow Dragon to “learn” from the misrecognition.

Instead of saying "scratch that" when you dictate text that is incorrectly transcribed by Dragon, a better choice is to select the misrecognized text (manually or by voice) and perform the correction (again, either manually or by voice) so that Dragon can learn from the error. Dragon will be less likely to misrecognize that word or phrase the next time around if you take the time to correct it. A direct command option to correct misrecognitions is “Correct XYZ,” where XYZ is the word or phrase you want to correct.