Good question. Market cap pertains to all coins in circulation. For instance, Bitcoin's market cap is Current bitcoin price x current total amount of coins minted.
Alot of the smaller alt coins have a very thin market. This means there is only a small amount of people actually actively trading the coin on exchanges. Since there is a lack of offers, a small amount of bitcoin can move the price in extremely large ways.
Yes the market cap rose by 64m USD but in reality, it only took a few thousand dollars to make it rise that high. If someone actually tried to sell for that price they would only get rid of a few before the price dropped back down to more sane levels, there just isn't enough liquidity in these smaller markets to move that much value in altcoins except for maybe the bigger alts (and even then it will be difficult without effecting the price majorly).
More than likely, if it took 250 btc worth of value to pump the price that high, it will take less than 250 btc for it to fall back to it's original price. There just won't be enough offers at the higher price to sell any significant quantity unless you get extremely lucky.
Just because the market cap rose by 64m doesn't mean there was 64m worth of USD value actually put into the market. In reality it could be only a few thousand dollars.
This is why market cap is considered a bad indicator for trying to figure out how much a specific blockchain should be valued.
I hope that makes sense!
