Double counting, and other errors
If you count First AND second languages, then you can't deduct the total from the whole world population. Many of these people are being double-counted.
Also, what happened to Cantonese? That has to be one of the most common Chinese dialects?
(and I doubt that 1 billion Chinese ACTUALLY speak Mandarin functionally. It's the official state language but many people would not actively use it until they move interstate.)