So many possible reasons starting with the most obvious :
1. Is it necessary to buy the exact replacement on a 1 to 1 basis ? If the elastic of one of your briefs starts sagging, will you buy EXACTLY 1 brief from the store?
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2. Maybe the purchase terms made sense to buy 12 aircraft. If during your earlier mentioned visit to the store to replace your solitary brief, would you not see that the pack of 3 briefs offer a better deal ?
3. The IAF plus HAL decided it makes sense to use this opportunity to beef up aircraft strength. Say you found out that the price of cotton and petrol will increase leading to costlier briefs, so why not buy now?
4. Lastly, would you be inspired by an OP who has more holes in his head than holes in his own ancient briefs , to draw such silly conclusions? How old are you anyway?