This index is being calculated since 2013 so when it started it was based on coins that were traded with large volumes on BTC-e and Cryptsy.
But price absolute values are extremely different so you need some coefficients.
Besides some coins are more important (Litecoin, Namecoin...) so more weight in % they have.
If traded on BTC-e and also on Cryptsy - more weight.
And % were fixed at the beginning. It was necessary to calculate ratios. That's about ALT19 index.
And there is also ALT100 index where 100 coins included with 1% weight each.