The contents of those scriptures have been retranslated countless times. Besides this fact, they were only written down about 500 years after Gautama Buddha died. Third, the words that were written down were those as transmitted by ordinary people, who had a huge gap in level of consciousness between them and Buddha, so they didn't understand exactly and didn't write down Buddha's exact words. And fourth, the scriptures have been changed by a number of monks who just wrote what they wanted in there. It's in these processes that the truth which Buddha taught was changed and has deteriorated through thousands of years.

You asked a very good question. Buddha, when he was Enlightened, also attained nirvana. What this means is that he saw the world of origin, where everything dies and is reborn again. This is why he could see that all the results are made by the causes. So, it's because nothing exists in the world of origin that Gautama said that there's nothing there. But Buddhist monks often misunderstand this as that when you attain nirvana you can finish all your suffering. However, Buddha never claimed that. He explained that when he attained nirvana he could be free from agony and illusions. But no creature while it is alive can be free from suffering. If we don't eat, we suffer from starvation. If we go to a cold place, we feel cold. So to cease suffering forever means that one would have to stop being reborn and die forever. The only way to free from suffering while you are alive is to disengage your consciousness from your body. This is what modern buddhism guides you to do through meditation. But it's the exact opposite way of Buddha's teaching. Buddha taught people to save themselves and to live forever, not to die forever.