Why is there a difference in the stock price during pre-market/after-hours sessions compared to when the market is open (during core-market hours)?

Why is there a difference in the stock price during pre-market/after-hours sessions compared to when the market is open (during core-market hours)?

The price during the Extended Hours market is the best price available across all the participating Electronic Markets, which may differ from the last close price during core-market hours due to changes in information available from news events, company earnings, or activity in foreign market