Uwe's comment on the question is spot on. The characteristics of the flow through the nozzle depend critically on the pressure ratio - the two pressures being the pressure at the entrance to and exit from the nozzle.
Above the critical pressure ratio flow through the nozzle is subsonic and it is not choked at the throat. Below the critical pressure ratio, the flow chokes at the throat, reaching a maximum.
This figure from The Dynamics and Thermodynamics of Compressible Fluid Flow by Shapiro shows the variation in properties based on the pressure ratio.
The most interesting feature of Fig. 4.3 is the maximum in the curve
of flow per unit area...The pressure ratio p/p0 where the
flow per unit area is a maximum is called the critical pressure
ratio and has a value for all real gasses and vapors of approximately
Pressure ratios greater than the critical correspond to subsonic flow
and pressure ratios less than the critical correspond to supersonic
The next graph from p. 87 shows the same parameters plotted against Mach number. You can see the knee in the area ratio curve at the critical pressure ratio well here. This corresponds to the throat of the nozzle.
There's a good discussion of the phenomenon here
If you lower the back pressure enough you come to a place where the
flow rate suddenly stops increasing all together and it doesn't matter
how much lower you make the back pressure (even if you make it a
vacuum) you can't get any more mass flow out of the nozzle. We say
that the nozzle has become 'choked'. You could delay this behavior by
making the nozzle throat bigger ... but eventually the
same thing would happen. The nozzle will become choked even if you
eliminated the throat altogether and just had a converging nozzle.
The reason for this behavior has to do with the way the flows behave
at Mach 1, i.e. when the flow speed reaches the speed of sound. In a
steady internal flow (like a nozzle) the Mach number can only reach 1
at a minimum in the cross-sectional area. When the nozzle isn't
choked, the flow through it is entirely subsonic and, if you lower the
back pressure a little, the flow goes faster and the flow rate
increases. As you lower the back pressure further the flow speed at
the throat eventually reaches the speed of sound (Mach 1). Any further
lowering of the back pressure can't accelerate the flow through the
nozzle any more, because that would entail moving the point where M=1
away from the throat where the area is a minimum, and so the flow gets
stuck. The flow pattern downstream of the nozzle (in the diverging
section and jet) can still change if you lower the back pressure
further, but the mass flow rate is now fixed because the flow in the
throat (and for that matter in the entire converging section) is now
The changes in the flow pattern after the nozzle has become choked are
not very important in our thought experiment because they don't change
the mass flow rate.