Coil springs do become "set" to length during the manufacturing process.
In the case of a Garand recoil spring, they then sit in a tiny tunnel up at the front of the rifle and become HEATED rather violently, especially by rapid-fire strings.
The result is the spring LOSING some of its initial "set" owing to it becoming LESS ELASTIC from the heat.
Like me when I read a silly post, it is LOSING ITS TEMPER... literally.
So it shrinks in length.
Length is CRITICAL for the reasons that PURPLE has explained.
Anyone who has ever rebuilt an old flathead motor knows about spring shrink, how to check for it, what the minimum and maximum length of a given spring should be. The same idea holds for rifles, except more so: you are holding that thing a few inches from your face.
Thanks, PURPLE, for the specs on the M-1 spring. Mine is nice and stiff, but I think I'll check it, anyway.