The math doesn't work like that. Those numbers aren't supposed to be the same ratio. That 500mil added to both accounts does not show that he lost value in gigas. If you add 500mil to each account, you are no longer supposed to be talking in ratios.
Let's pretend the gigas aren't halved.
If both accounts grind at the same speed and each earn 500mil, the ratio is now 1,500,000,000/514,000,000 = 2.91828793774. Not the same ratio.
You have to multiply each amount of gigas by a constant to get the same ratio, not add the same number to both if you want the same ratio.
And you meant 1,000,000,000/507,000,000 = 1.97238658777.