Nasdaq Market Hours Pre-market Trading On NASDAQ?

Pre-market trading on NASDAQ? - nasdaq market hours

Hello

Officially, the extended hours trading on the NASDAQ Stock Market, 7.00-9.30 and 4.00-8.00. However, running when the pre-quotes are gathered in the actual operations, even before 7:00. Why and how these ECN pre-market trading will be extended from trade with CER in the various official calendar?
Thanks