[DECtalk] Why are there so many variations of Dec Talk Sound?

[Brent Harding]

It seems these days I've heard more variations of Dec Talk than I ever did 
in years ago. I know 5.0 is where they messed it up, but it seems that 
various modern implementations sound different than others. Why, for 
instance, does Mobile Speak seem to be the worst, Window-Eyes isn't too bad, 
and other modern products fall somewhere in between? Are device and software 
manufacturers able to, and are, changing the way it sounds? I understood 
there to be 2 different ways Dec Talk sounded, but I believe I have heard 
more variations.