Here is something I read:
Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator. In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR).
It seems that the "field current not being affected by load current (hence constant output voltage)" seems to contradict the next line ("In actual use...").
Why is armature resistance increasing with increase in load?