Comparator not outputing redstone signal from minecarts with chest (or hopper)

I don't know if it's a bug or not but I think it worth asking.

What I expected to happen was:
As comparators outputs a signal from filled containers, I thought it would work the same way with minecarts with chest and with hopper.

What actually happened was:
Even if the minecart with chest (or hopper) is full of items the comparator doesn't output a redstone signal.

Steps to Reproduce:
1. Place a rail.
2. Place the minecart with chest above the rail.
3. Fill the chest.
4. Place a comparator next to the minecart as you would do with a normal chest.