Here's why:

* C4 Plants: C4 plants use an enzyme called phosphoenolpyruvate carboxylase (PEP carboxylase) to fix CO2 into a 3-carbon compound called phosphoenolpyruvate (PEP). This reaction forms oxaloacetate (OAA), a 4-carbon compound.

* CAM Plants: CAM plants also use PEP carboxylase to fix CO2 into PEP, resulting in the formation of OAA. However, they do so at night, storing the OAA in their vacuoles. During the day, OAA is decarboxylated to release CO2, which is then used in the Calvin cycle for sugar production.

While both C4 and CAM plants initially fix CO2 into OAA, the subsequent steps in carbon fixation differ significantly due to their distinct strategies for minimizing photorespiration.