While a character is in the air, hitting enemies increases a hidden counter that decides how many points to award. The first few enemies are worth 100, 200 then 500 points, then the next few are worth 1000. Eventually, enemies are worth 10000 points.
The counter resets when a character lands back on the ground. But if enemies are hit with a rolling attack and the player never goes off the ground, the counter never resets - rolling back and forth in areas with regenerating enemies such as the ink-stamp rabbits in Press Garden Act 1 can let the player max out their lives counter at 99 quickly by collecting vast amounts of 10000 point awards (an extra life is awarded every 50000 points).