Custom build recoil spring weight

I have an AccuracyX pistol that came from the factory with a 12.5 lb recoil spring. I thought this was odd because all the recoil springs I had seen up to that point were in full pounds and not 0.5 lb increments. 

Since then, I have come across Wilson Combat springs that are only available in 1.0 lb increments EXCEPT they make a 12.5 lb spring.  

Is there something magical about the 12.5 lb recoil spring weight? 

Why is that option available in a 0.5 increment?  

Seems overly specific compared to the other spring weight options.

I have lately been messing with recoil and other springs. The weight of the spring is supposed to be at the compressed length. Online drawing of the GI 1911 has 1.81" listed. Doing the math and also measuring the gun I find that the distance the slide travels to slightly beyond lock back notch allowing the gun to lock back after the last round if fired is 1.935" That corresponds with the 1.81"compression length.  The compressed length I got from Wolff is 1.625" That works out to be at the maximum the slide can travel maybe slightly more. Probably taking in manufacturing tolerances. It's possible other makers are using a different compression length at which measurement it taken.
 To get different compression amounts as felt by the slide in less than 1/2pound rates changing the main spring, hammer and bottom of the firing pin stop could be a way to do it.
 I like on my guns to use as heavy a recoil spring as possible with the load being used and to have as low as possible effect on recoil by the hammer. That's because accuracy is dependent on the closed slide!