Specific Impulse & Exhaust Velocity

Hi all,
When are the Specific Impulse of a rocket and the velocity of its exhaust
gas not the same?
(OR when are they they same, if that's simpler)

Seems to be that one of the assumptions make is that the nozzle exit
pressure is very close to the combustion chamber pressure, though I don't
know why.

Makhno wrote:

When are the Specific Impulse of a rocket and the velocity of its
exhaust gas not the same?

Huh? Specific impulse is measured in seconds - exhaust velocity is measured
in meters per second. They can never be the same!

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I øvrigt mener jeg alternativ behandling bør forbydes.