What I expected to happen was...:

I expected a hopper minecart to pick up dropped items from the block above it, and only the block above it.

What actually happened was...:

It picked up items from one corner of the block above it (see picture attached), but it also picked up dropped items from corners of blocks around it as well.

From testing, it appears that the hopper minecart is centered at its current position + 0.5 in both x and z. This theory also supports the evidence that it doesn't pick up items from anywhere on the blocks, but instead just from the corners where the minecart is centered. (see pic cause I think I just confused you more)

Steps to Reproduce:

1. Place a hopper minecart on a rail

2. Place blocks above the minecart

3. Toss items on the blocks and it will reproduce the results above