It has always been an issue with getting the perfect spring rate, so introducing a 25lb increment change enables the rider to include the spring a part of their tuning package thus buying two springs for different types of riding or surface i.e. you would choose to go 25lb lighter for more high-speed descent riding where compression values increase their range thus not relying on the spring so much support.
 
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If it is a Rock Shox Super Deluxe here is some useful additional information for you. There is also a short video on how to install your pom spacers and spring. 
One thing to note is that Rock Shox Super Deluxe usually are not at the 250psi in the IFP chamber that they need to be as the dampers come finger tightened from the factory and usually lose pressure between the factory and being mounted on the bike. It is something you should get checked. You can take it to a shop or do it yourself but you need a valve core remover. Usually, the core comes out because the machine that assembled the damper doesn’t do it up properly which is why it loses air pressure anyway.