That is caused in part by the difficulty of understanding words in different accents and tones of voice. The vocabulary necessary to run a telephone directory system, however, is relatively small and the computer is able to guess at the name and get it right a good part of the time. That is not so when you are dictating sentences using a vocabulary of 30,000 or more words. Here the complexity is enormously multiplied and the possibility for error is extremely high. Two of the most popular ways of dealing with that problem made voice recognition difficult and unpleasant in the context of dictation.
Discrete speech is the careful enunciation of each word in a staccato style.
And, as a result, the error rate increases dramatically. One way of dealing with that error rate poses the second major problem that the older voice recognition systems had. If you talk long enough to a computer and you constantly tell it when it makes a mistake it will eventually build a pretty reasonable capability for recognizing your voice. The problem is that this training may take many hours or days and is incredibly boring. Even then the number of errors in the older systems was so high that there was no real time saving.