Butterworth Polynomial from Butterworth Filter
The denominator of the transfer function of an analog Butterworth filter is a (non-normalized) Butterworth polynomial. Also the numerator is just a constant (i.e. the coefficients of higher order are 0).

You are designing a discrete filter here, so this does no longer apply. Your numerator has three non-zero coefficients, and the denominator is not a Butterworth polynomial.

You can check that if you design an analog Butterworth adding the parameter 's' when calling the function, you will indeed get a one non-zero element array B and the coefficients of a non-normalized Butterworth polynomial in A.