Saplings need 2-9 bonemeal when in a 3x3 space?

When I was within a radius of 1.5 of a sapling (3x3) whenever I would use bonemeal up to 9 times. However If I'm further than that or right in the sapling, it only goes up to 5 times.

There could be more patterns but I'm not bothered finding them out.

Also why is it so high? This breaks sky survival maps.