Colonial Supplies Executive Order Minor Issue

I wanted to check-out the Colonial Supplies Executive Order in Supernova (first screen snapshot below).  It says that the effect will be a 50% reduction in the decay from our colonies (I still think that the effect should show as -50% not +50%).

I took a screen snapshot of my Castor II colony before I executed the Colonial Supplies Executive Order and it shows a decay rate of 51.5% (second screen snapshot below).

After I executed the Colonial Supplies Executive Order I took another screen snapshot of Castor II (third screen snapshot below) and it now shows a decay rate of 26.5%.  Mathematically, .515 * .5 = .2575 or 25.75% not 26.5%.

Can you please explain how the new decay rate of 26.5% was calculated since it is clearly not a straightforward multiplication of 51.5% times 50%?  I know it is not off by much, but my math training makes it annoying when I cannot duplicate the calculation.