Because they're shiny, obviously! :P But they really do have a different foil pattern from other cards. They've got 3 foil sideways arches behind the text of the card.
Salamence from Arceus is pretty freaking awesome by itself. If there's one Lv. X card worth playing for fun though, it's Salamence Lv. X, as it's got a pretty cool Poke-Power:
I ran a deck with 2 Salamence and 2 Salamence Lv. X. Somehow I ended up with 5 Salamence Lv. X in all the boosters of that set I bought, only getting Tangrowth Lv. X of all the other Lv. Xs and none of the 3 shining Pokemon cards in the set.
I wouldn't count any of the World's deck cards because they lack the holofoil shininess of the original card. I don't have a clue what other shiny version Pokemon I have in the cards other than those 3.
I have a brand I would recommend. What color are you looking for? You can get pink, purple, red, green, gold, brown, blue, black, orange, white, and... several other colors, I think. You can also get the official patterned sleeves from the Pokemon TCG, but not in the brand I'm about to recommend. The brand I recommend is Dragon Shield.