In my understanding and experience, such questions tend or risk to open a can of worms on users forums and to meet the wall of trade secrets among winders.
Now, the following page might give you some thoughts to meditate about the sonic properties of alloys and the reason to use 1215, once its advertising content put in perspective: https://www.pafhumbucker.com/paf-pickups-screws.html
[It's not that the whole discourse is consensual, though: about the differences between vintage and modern steels, for example, some winders would strongly disagree with this webpage...]
Many times its actually the opposite, lesser alloys tend to be overly bright and harsh. The preferred alloys tend to have an even pleasing response to the ear.
Sound isnt the only consideration when manufacturers choose what alloy to use. Cost and corrosion resistance factor in also.
You have to demo screws side by side to appreciate that there is a difference but it is small. But a pickup is a sum of parts, if the screws and slugs are less than optimum and so are the bobbins the overall quality of sound can be effected. If you have a pickup you are trying to tweak a screw alloy change will rarely do it. Changing style and length might though.
The tonal outcome of these differences has already been expressed by the other contributors: lower 2 last digits = less carbon = less brightness.
Now, and as also suggested above, alloy is not the only parm to consider with magnetic steels in guitar pickups:
-the actual ferrous mass of slugs/screw poles/blades/ keeper bars matters, since it affects the inductance (more inductivity= more power and mids)...
-The shape and location of these parts proportionally to the coil(s) and mag(s) contribute to more or less Foucault (eddy) currents rounding the tone...
-their magnetic properties may vary after further physical treatments (annealing, plating)...
Etc.