Back in the good 'ol days of R/G (That's R/B for us not-Japan residents) the Game Boy Brick cartridges could only hold so much data. To save space, the answer was to refference the same audio for different Pokemon cries. The problem with this is that it wasn't expected to explode into popularity, enough so that if they were to change the cries later, every poke'fan in the world would be able to point it out. Now, of course, there is far enough space for each Poke to have thier own sound, and will no longer need to be recycled for another Pokemon.