The first time I ever played with software speech synthesis on a microcomputer (not hardware synthesis, like in TI's Speak & Spell) was around 1983, using S.A.M for the Commodore 64.
A year later, I remember hearing the speech synthesis from the Macintosh unveiling, and being struck by the notion that it sounded the same as S.A.M on my C64. I suppose I thought that the SID chip and whatever the Macintosh used for sound would lead to much different results. Two years later, I got an Amiga, which came with built-in speech synthesis just like the Macintosh. Same. Voice. Again.
But the biggest surprise was the first time I heard Stephen Hawking speak. This just didn't seem right, since Professor Hawking should obviously warrant something far better than a "C64 under-the-bonnet" for his translator.
Additionally, the modern (2013) SpeakJet synthesizer chip also seem to have the same voice, while certainly being far removed from the 1980s hardware above.
Why do they all have that same strange voice?