Why did human lifespans drop after the Flood?

In [Genesis 5](https://www.biblegateway.com/passage/?search=Genesis+5&version=ESV) , a brief account of the lives of Adam's descendants is narrated. All of them had long lives, for instance, Adam lived 930 years, Seth lived 912 years, etc. But in [Genesis 6:3](https://www.biblegateway.com/passage/?search=Genesis+6%3A3&version=ESV) , God seems to shorten the lifespan of man to 120 years or at least that's what I understood reading that verse. **Did I understand it right? How is this fact explained by people who consider Genesis to be literal history?**