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.
Unable to reproduce. There was no noticeable difference in testing. Keep in mind that the rates are random, and you may have just been unlucky.