I can't get too worked up over this.
For one thing, there are countless languages and dialects in the world today where similar words get pronounced differently. Take a name like José. Spanish speakers tend to make the J sound closer to an English H, while Portuguese speakers will use a more English-like J.
Moreover, pronunciations evolve over time. A lot. My own surname centuries ago sounded nothing like how I pronounce it today.
Diablo is it's own world which may or may not be connected to ours. Usually, fantasy narratives are loosely based on the past, though I was reading some Terry Brooks and discovered his fantasy realm is actually post-apocalyptic Earth. Kind of clever how he tied that in. Anyway, all I'm saying is that both distance and time can distort language quite a bit.