Why glitchy symbols occur in names

In early Pokemon games, tiles are referred to as numbers. Text is part of the tile set so when a Glitch Pokemon's name is viewed, the numbers in its name are somewhat random so parts of the tile set are included.

This explains a little why you can't get Mew with the Old Man trick. Mew's character is number 21 which is a tile which is not included in the name enter screen. There is a way to encounter a Mew with the Old Man trick with a Game Shark by changing the Pokemon encounter name portion in the player's name to number 21.