Let me explore. Some are obvious things. Others need to be further explored.
We had a printing company print 100,000 stickers and even between this
batch there were slight changes of hue
First, we need to really explore if you had a change in Hue
Hue is a change in "color", you printed red and it turned into blue. There is no way this can occur if you used one spot ink. I do not mean "Pantone", I do not mean CMYK, I mean one direct ink.
The only way this could happen, is if the press was a bit contaminated with a previous ink, or at some time, they run out of the prepared ink and need to improvise a bit more.
Normally you prepare, for example, one kilo of a special colored ink assuming this will be enough for the entire project, based on the design (amount of area of the color), size of the paper, absorption of the paper, etc. You will always aim to have more ink than you actually need, but not that much extra because ink is expensive.
No "Pantone" involved yet.
When you have one spot ink it can vary on the press in different ways, mainly the amount of ink, the "density" of the ink. Just as some examples:
- The design could need a lot of ink in one zone, leaving little time for the rollers to recharge the ink.
- Different pressure of the rubber roll
- Adjustments on the valves
Nothing to do with Pantone yet. Also, no "hue" change, just changes in density. Let's say changes in "value" or transparency if you will.
Ok, let's talk about Pantone now.
If Pantone is supposed to be printed the same color no matter who prints it
You do not print Pantone. You print a color ink prepared according to some formula (a combination of inks), based on some basic colors, manufactured by different people. Pantone is what gives you this formula and gives some manufacturers of inks the "authorization" to declare some of their base inks, as "Pantone".
A lot of steps here.
Different manufacturers with different products, some probably give the ink the advantage of dry faster or more oily, or alcohol-based, ecological... who knows, therefore there can be a slight change in the base color.
But the most important point is:
Who prepares the final color?
If a run is big, and I mean big, you can send to manufacture a good amount of ink, let's say 5 kilos. The ink will be prepared in a controlled environment and you will receive 5 kilos of uniform ink in a couple of days.
But 100,000 stickers are not that much. Let's say you have a grid of 10x10 stickers on one sheet of paper. You are only printing a basic 1000 sheet project. You can print it with 1/4 of kilo or less. The printer will not pay 20 times more for ink. It will prepare the necessary ink "by hand", yes, they probably will use a weighing machine, but what if one ink has some solvent evaporated.
They will try to match "by eye" looking at a simulation of the print not putting the ink in the machine, but just smearing some of it on a paper... Got the idea?
If you want really consistency you need to ask for your project a brand new set of Pantone ink jars, and work with the same print company... and live with the idea that you can not have the exact same color on every print.
What Pantone offers is not "perfection", but rather a somehow reliable color guide, with some standardized basic color inks that can be reproduced by different people with a low margin of differences if processed in a somehow standardized fashion.