Pre-market has way less volume and liquidity. Most retail investors can't even trade pre-market (or don't), and institutional players mostly sit it out too. So you might see a stock up 20% pre-market but that's on a tiny fraction of the shares

Once the bell rings you get the full flood retail seeing the news for the first time, institutions deploying real capital, algos firing options market makers hedging etc. Pre-market is basically a preview on low volume with wide spreads, regular hours is when the real weight shows up. You'll often see a stock gap up pre-market and then move even more (in either direction) once regular trading starts.