Is the bound achieved by his proof actually 70000000? That looks like a suspiciously round number. Why can't they tell us the exact bound that he achieved? (Presumably some slightly smaller number?)
It _could_ be that some super-expensive calculation is required for each candidate bound, and Zhang guessed 70000000 as a starting point and did the calculation for that number, which succeeded, but he didn't bother repeating it for any smaller numbers because it would cost him too much. But if that was the case, that would be interesting in itself.