The second % represents the cumulative percentage of total Bitcoin supply held by that category and all categories above it. It adds up the %s as you move up the table, showing the total share of Bitcoin held by smaller balances combined with larger balances. (i.e. Shrimp (0.1 - 1 BTC) shows 5.42% (individual category) and 98.41% (cumulative percentage, including all categories above it.)