Documentation Link

https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.boxplot.html

Problem

Documentation says that

The whiskers extend from the box by 1.5x the inter-quartile range (IQR).

This is clearly not true as in default configuration upper and lower whisker can have completely different length.

You can try the following code for a very visible example.

import matplotlib.pyplot as plt
plt.boxplot([-4.7 , -1.29, -5.78,  2.73, -5.54])

I managed to track the source of this behaviour to relevant lines in cbook (https://github.com/matplotlib/matplotlib/blob/c23ccdde6f0f8c071b09a88770e24452f2859e99/lib/matplotlib/cbook/__init__.py). I have attached relevant lines with my comments below

# 1.5 * IQR below Q1 location
loval = q1 - whis * stats['iqr']

# select points that are above loval
wisklo = x[x >= loval]

if len(wisklo) == 0 or np.min(wisklo) > q1:
    # there are no points above loval (pathological case)
    # OR
    # points above loval fit inside the box
    
    stats['whislo'] = q1
    # set whisker length to zero
    
else:
    # there are some points in the range [Q1 - 1.5 IQR, Q1]
    
    stats['whislo'] = np.min(wisklo)
    # put whiskers end exactly at the lowest point

Suggested improvement

I suggest making the relevant description more accurate. Replace current wording:

The box extends from the first quartile (Q1) to the third quartile (Q3) of the data, with a line at the median. The whiskers extend from the box by 1.5x the inter-quartile range (IQR). Flier points are those past the end of the whiskers

With a slightly longer but correct description:

The box extends from the first quartile (Q1) to the third quartile (Q3) of the data, with a line at the median.
The whiskers extend from the box to the farthest point within 1.5x the inter-quartile range (IQR) distance. Flier points are those past the end of the whiskers