There are a lot of good guesses in this thread but also some inaccuracies
While generally order size and timeline are the main factors, there are a few other things that can affect the order in which chips are ultimately shipped.
Inlays - there is only one inlay punch - each time the shape or size has to be changed the punch has to be stripped down and set up again. Like many processes this is time consuming so therefore you do this when you have a 'batch' to do on a particular size or shape. This won't affect most people but for those that place their order early but don't supply artwork until the last minute obviously this can have an impact. It's the main reason we print inlays pretty much as soon as proofs are approved.
Clay - as most know it doesn't have an infinite shelf life and there is a maximum you can make at any one time anyway (purely due to weight). It generally takes a day to roll one color and it's not always possible to roll every day. So, if at a particular time your next batch of orders happens to include all 38 colors it can take 10 weeks to cycle through everything once. So obviously that also has an impact.
Edge spots - It takes some skill to punch and assemble the smaller and complex spot patterns, whereas anyone can put together a 312 for example. So a larger order with small and/or complex spots could literally take 4 months to put together compared to a week for something small and simple.
Rejects - again as most know, we have to start by making more pieces than the finished order calls for. That has to be estimated based on experience. Not all of the extras are taken right through to the finishing stage unless needed. So lets say 2 orders are finished, ground and inspected at the same time. One has sufficient good chips to be shipped so out it goes straight away. The other order is short. The extras are finished (which could take as long as another week) and it's still short by just a few. Those have to be made from scratch so add another week. Then a couple of days until it can be fed into the pressing between other things and all of a sudden it's 3 weeks later going out than the other order that was inspected on the same day. During that 3 weeks a few other orders that were placed later could well have been shipped out.
Spot color changes (for example) - when it's not too late we accommodate these but obviously if we were just about to make your parts when this happens, and we don't have the clay on the shelf for the new color then you are going to be affected by the 'Clay' section above. Many people forget that although they placed an order '9 months ago', it took them 4 months to send the art and then they changed the colors twice. I'm not thinking of any specific orders here but it probably happens as much as 1 in every 4 orders.
Molds - right now the FDL and DIECARD are in. Diecard is rarely mentioned as it's not often ordered but there were a couple of very old orders that had to be done.
FDL should be done in a couple of weeks and that will be followed by the Crowns (has to be Small followed by Large as the outstanding orders on the small go back much further. Yes the crowns will be a long run, 30 odd orders for 40,000 chips is an absolute minimum of 3 months, probably more like 4 by the time the large are complete.
DIECARD will be a few weeks later changing to NO MOLD for a single medium size order (no choice, it's been waiting too long) followed by BDIAMOND.
As that all takes us out a fair few months nothing else is fixed yet.