The area hopper minecarts will pick up items in is not centered.

A minecart with a hopper will pick up items that are up to a full block above it. However, some items will not get picked up. This is because the area is centered on the Southeast corner of the hopper minecart. Items that land in the red area in the picture below will get picked up. But, items that are outside the area, such as the brick in the picture below, do not get picked up. I was facing south when I took this screenshot. This does not happen with normal hoppers.