I don't think no-one's noticed this yet. Like what Mewbiraxys has pointed out, I've also noticed a pattern.
The male characters (Red, Gold and Ruby) chose Bulbasaur, Cyndaquil and Mudkip as their starters. The male rival characters (Green, Silver and Wally/Emerald) chose Charmander, Totodile and Treecko. And the female characters (Blue, Crys and Sapphire) chose Squirtle, Chikorita and Torchic.
My point is that all the main characters have chosen one of each type of starter. For Kanto it's Grass, Fire and Water; for Johto it's Fire, Water and Grass; and for Hoenn it's Water, Fire and Grass. It forms a balanced cycle.
And notice this, Green, Silver and Wally/Emerald have chosen the starters in the Dragon Egg Group. Once it goes back to Grass, Fire and Water for Diamond and Pearl (presumably), the male rival character will get the Dragon Egg group starter.