Why does scipy.signal.ShortTimeFFT.upper_border_begin depend on the window values?

I'm having some trouble understanding some behaviour of the lower_border_end method of the scipy ShortTimeFFT. Everything from the example is clear to me, but I don't understand this:

ShortTimeFFT(np.ones(6),2,fs=1).upper_border_begin(50) >>> (45,24) # Just as in the example ShortTimeFFT(scipy.signal.windows.hann(6),2,fs=1).upper_border_begin(50) >>> (47,25) # Same window size but different result ShortTimeFFT(np.hstack(([.5],scipy.signal.windows.hann(6)[1:-1],[.5])),2,fs=1).upper_border_begin(50) >>> (45,24) # replacing the 0. border weights of the hann window brings us back to the example result

Why does the window values affect the upper_border_begin?

Thanks!