Could be based around the transaction fees.
Things with a transaction fee are given priority processing. I'm guessing they don't add anything above the minimum fees for the transaction. Meaning it gets a low priority compared to all the smaller transactions at minimum fees, as they represent a larger percentage of the total transaction.
My guess is that they didn't add ANY fee. Does that mean that transactions without any fee has the potential to be delayed substantially?
) but i know that it will go in to a block. It is just a matter of time when that will happen.