BlackJack: Why Do More Decks Increase the House Advantage?

[TomBrooks]

In Blackjack, why does playing with more decks provide the house with a better advantage? I don't see why it should make any difference. (Ignoring any card counting factors.)

The Wizard of Odds provides a different strategy charts for One, Two, and Four-or-More Deck games. But why would the strategy be different? While there are more cards, the relative number of any particular card to the total number of cards remains the same.

For one deck, Woo provides a Correction page for changing the action on certain specific holdings. I can see where that provides an advantage.