They're known by a few different names. Non linear pitch springs, irregular pitch spring, etc.
Supposedly non linear springs don't follow hookes law (I got this from googling), "k" can be variable and you require differing amounts of power to compress. Not to worry just install the spring with tighter coils on spring guide and you'll be fine.
Also, it's well documented so if required check this thread out: http://www.airsoftcanada.com/showthread.php?t=37229