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→first case it setting default size of 10(your case numpy setting ~26), if bin size not given,

→ Second case you are allowing, your array to fit in the range(min and max) if the arrary is not sequencial.

More explanation here:

a : array_like

Input data. The histogram is computed over the flattened array.

bins : int or sequence of scalars or str, optional

If bins is an int, it defines the number of equal-width bins in the given range (10, by default). If bins is a sequence, it defines a monotonically increasing array of bin edges, including the rightmost edge, allowing for non-uniform bin widths.