Because the glucose in capillary blood is not fully delivered to the cells yet.
Once the blood leaves the capillary and enters the vein, the glucose has then been delivered to the cells and the blood is considered used.

(1) in the fasting state : can generally be ignored
(2) after a meal or glucose load
① capillary : about 8% higher than venous samples
② plasma : 14 ~ 16% higher than whole blood