It's being applied incorrectly, though. You'd have to derate all fuses by 45% for it to be correct.

Using your above example of 2x25 amp fuses wouldn't work out to be 37.5 amps...you have to assume both fuses only pass 75% of their rated current, and then derate another 20%, so they only pass 55% of their rating, so 2x25 amp fuses works out to 27.5 amps, because you don't know which fuse (if either) is incapable of passing enough current to cause an open.

Another example: 3x40 amps (like the picture) works out to be 40 x .55 x 3 = 66 amps. You can also add all three together and then multiply by .55 to acquire the derated ampacity. Using the linear 50% derating as you were told works out to 70 amps (40+20+10). It's close, but that 4 amps can mean the difference between blowing fuses or not. I also like the simplicity of just multiplying the overall load instead of always halving things.