in stoichiometric defect how can interstitial defect not affect the stoichiometry of the cell

Interstitial defect: This type of defect is caused due to the presence of ions in the normally vacant interstitial sites in the crystals.The defects do not disturb the stoichiometry (the ratio of numbers of positive and negative ions) is stoichiometric effect.

Because interstial effect generally occurs in non-ionic solids where cation and anion won't be present only atoms or molecules of covalent bond (eg H2, O2) is present here. So eventhough one extra h2 occupies the interstial, the stoichemetry of the hydrogen is gonna remain the same i.e one H is gonna be bonded with only one H throughout the solid structure. Compound where their stoichemetry prone to be affected are mostly ionic compounds. But in such compounds, both interstial and vacancy defect takes place called as frenkel defect or dislocation defect so as to maintain the charge neutrality and stoichemetry.