"all his wealth" would mean ALL of his wealth, which is unrealistic. Saying "99.999%" of my wealth invites critics later on when he donates "99.98%". Substitu

"all his wealth" would mean ALL of his wealth, which is unrealistic. Saying "99.999%" of my wealth invites critics later on when he donates "99.98%". Substitute "virtually" by "almost" and it's the same. It's just style.