The confidence intervals of plot_reliability_diagram are made a bit too wide by

# We make the interval conservatively monotone increasing by applying
# np.maximum.accumulate etc.
lower = -np.minimum.accumulate(-boot.confidence_interval.low)
upper = np.maximum.accumulate(boot.confidence_interval.high)

This is not conservative because in might lead to the conclusion that the model is auto-calibrated - within the uncertainty - while it actually is not.