The soul goes to barzakh, how did Muhammad (saw) see people in Jannah and Jahannam?

We are told that when people die, their body stays in their grave and their soul goes to barzakh where they wait for the day of judgement. After that people enter either Jannah or Jahannam. However during Isra' wal Miraj, the Prophet (saw) traveled to Jannah and Jahannam. I understand that the Prophets are in Jannah already as there may be an exception for them. However Muhammad (saw) also saw people suffering in Jahannam too. So my question is how or why were there people already in Jahannam when the soul waits in Barzakh after death?