[Python] Why does df.isnull().sum() work the way it does?

When I do df.isnull().sum(), I get the count of null values in a column. But the default axis for .sum() is None, or 0 - which should be summing across the columns.

Why does .sum() calculate the sum down the columns, instead of the rows, when the default says to sum across axis = 0?

Thanks!

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