When plotting summary vectors for which observations have been 'installed' with the Observation config, ERT will plot the observed values. If the number of observations is less than Errorbar max, ERT will use errorbars to show the observed values, otherwise it will use two dashed lines indicating +/- one standard deviation.
To ensure that you always get errorbars you can set Errorbar max to a very large value. On the other hand, setting Errorbar max to zero will ensure that ERT always plots observation uncertainty using dashed lines of +/- one standard deviation.
The setting here will also affect the output when you are using the TEXT driver to plot.