Unexpected intersection bounds and areas

[tksharpless]

I'm using Clipper2 to intersect simple, non-convex closed polygons (typically with 10 to 100 float vertices), and OpenCV contourArea() to measure them.
I've noticed that the calculated area of the intersection is sometimes as much as 3% larger than the area of the smaller input polygon, and that its bounding box is often slightly larger that the intersection of the bounding boxes of the inputs.
Is there a reasonable explanation for this?

[reunanen]

Do you have enough precision? See e.g. this.

If that doesn't help, then maybe you'd like to share some code and/or data? (Ideally, a minimal reproducible example.)