"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.

Because keyboard-warrior pedants will moan if he bought himself a $200K Porsche (0.0001% of that $200B) or even just a dinner for his kids...

I can totally see here the headline "While millions sleep hungry in Asia and Africa Bill Gates throws away uneaten fries after dinner at a local restaurant like its nothing"

Because there will probably be enough left at the end that for most would be wealthy beyond their richest dreams.

It's hard to get by with only 1.08 billion dollars…

Not that I think giving away most of his money isn't admirable. But I think he'll still qualify for "he died rich".

Because "20 years to give away all my wealth" would be misleading? He plans to retain some of his wealth.

Not to mention it's pretty difficult to give away all money. Some of it will need to pay for the administration of programs in existence after these 20 years, accountants, and of course himself - if he's still alive by then.

Maybe because he promised his kids a few million, or fail to give it away on his terms, and he doesn't want to be nagged about it.