What happens to unclaimed Open Assets (colored coins) asset quantity units?
I had the exact same question when reading over that specification document. I'm not sure if that is a mistake in the example, but the conclusion I reached is that the colored coins are destroyed. 

I tested this with the Python Open assets library and the transaction that claimed less than the full possible number of assets was still a valid colored coins transaction.

So, unlike normal coins where the 'leftovers' are given to the miners, unclaimed open assets are destroyed.