The student used the following device to determine the percentage of oxygen in the air. Experimental steps:

The student used the following device to determine the percentage of oxygen in the air. Experimental steps:
① First, clamp the rubber tube with a clamp;
② light red phosphorus in a burning spoon;
③Insert a burning spoon into the jar and put a cork on it;
④Open the clamp after piercing.
After the experiment, the measured oxygen volume was found to be less than 21%.
Q: What are the possible reasons for this?