Creatinine clearance slightly overestimates GFR because creatinine is moderately secreted by PCT
Most absorption ad secretion is done at the PCT. It is this reason why the PCT is the most metabolically active portion of the nephron.
As this image shows, you can see that reabsorption & secretion occurs at the PCT
this means that both PAH & creatinine are secreted for elimination at the PCT along with the additional volume freely filtered through the bowman's capsule.
Furthermore; Inulin is neither secreted nor reabsorbed.
hence Inulin is better for GFR. But, creatinine is normally used for GFR as an estimate (probably d/t ease)