Volume By Cross-Sections Help

We have a project where we have to make a model of finding volume by cross-sections, and I'm somewhat confused. This is the problem:

Find the volume - base bounded by y=x+1 and y=x^2-1, cross-sections are semi-ellipses of height 2 (perpendicular to x-axis)

And I have the formula piAB/2, A=height and B=radius. I subtracted the top function minus the bottom to get the base, and divided that by 2 to get the radius, but I don't really understand the height part. Why is the height given a constant? Shouldn't it change throughout the index?