Why does setting the horizontal scroll bar range for the first time also set the vertical range, and vice versa?

A customer observed that if a window has never set any scroll bar parameters, then it reports its scroll bar range as nMin = nMax = 0 in both horizonal and vertical directions. But once you set the (say) horizontal scroll bar range to anything (say, nMin = 0, nMax = 999), then the vertical scroll bar range reports itself as nMin = 0, nMax = 100.