CFM is not a static value it depends on the 'load' applied to the fan, a fan with no restrictions will have higher CFM than the same fan facing no restrictions and the relationship between the CFM and the static pressure depends on the fan design. Low static pressure fans can't push air effectively through tubes or restrictions and the flow volume (CFM) drops because it cannot push the air on the output hard enough to flow. High SP fans usually have larger more closely spaced blades so that the air cannot escape back through the fan.

For example on radiators you need higher static pressure fans to push the air through the small space between the fins on the radiator, if you use a high CFM fan with low mm H2O (static pressure) the fan doesn't actually manage to push the rated CFM through the radiator. Same for ducting you see in 3DP applications.