There are a multitude of factors to consider with regards to the piers and the prioritisation.
As Steve regularly says, when users submit submissions ideally they should be items due to start rising soon, and some items from different parts of the sky to take into account different blocks of time throughout the night. Such as dusk/dawn
The algorithms in place determine which items have more chance to grab the required data whilst taking into account any moon avoidance/moon down parameters and other filter priorities.
This can lead to newer items being finished first due to the system giving them priority as it may calculate that the item would be difficult to image further down the line.
We have seen and proven that the system yields an enormous amount of data when utilised properly.
The piers will never do jobs in the numerical order that they are input. It will always strive to get the most productivity it can each night and that means the data collected will be spread across multiple jobs. Before you know it, you have 5-7 completed jobs out of nowhere.
Steve or Pete can elaborate if I have missed anything.