Minecraft 1.8.3, Minecraft 1.8.7, Minecraft 1.8.8, Minecraft 15w35b, Minecraft 15w37a
Java 1.8.0_31 64bit
A hopper split it's items into a minecart with a chest that's in either the -z (north) or the -x (west) direction and the one below it.
If minecarts with a chest is placed in a 3x3 like in the image below the four minecarts in the north-west (-z & -x) corner have split the items.
The split isn't an even split, it can be random but usually it's about the same amount in each minecart.
The bug was first noticeable in Minecraft 1.8.2-pre5.
A world is provided where I have done my testing.
1. Place two tracks and block of the ends so no minecart can derail.
2. Place a hopper over the rail that's either towards the positive x or z depending on the direction you placed the tracks.
3. Place two minecarts with chest side by side on a track.
4. Place items into the hopper and watch the minecarts inventories.