Many of the cryptocurrencies (at Cryptsy for example) are not traded directly for USD. Instead, they are purchased with bitcoins or litecoins and so the numbers you see on CoinMarketCap.com reflect the percentage price change in each coin in terms of the dollar value of the number of bitcoins or litecoins that it purchases.
For this reasons you'll often find that a good day for bitcoin means a good day for other altcoins since, for it not to be a good day for these other altcoins, their value relative to that of bitcoin would have had to have declined by a percentage sufficient to cancel out the gain in the value of the bitcoin (e.g. if your cryptocurrency remains flat at 0.1BTC/coin and bitcoin goes up 10%, your cryptocurrency still trades at 0.1BTC/coin and so you still have the equivalent of 0.1 bitcoin times the number of coins that you have but it just so happens that your coins are now more valuable since bitcoins are now more valuable).
In Coinmarketcap.com, does anyone know what the column "% Change (24h)" is based upon? Is it the percentage price change in each coin in terms of USdollars or or in terms of bitcoin equivalent or in terms of market cap share?
