Why is the allotment value for my T-bill purchase lesser than the face value of ₹100?

T-bills do not carry an interest component. This is one of the biggest differences between T-bills and Bonds. T-bills are issued at a discount to their true (PAR) value, and upon expiry, they are redeemed at their true value.

The true value (also called the par value) is ₹100. This T-bill is issued at a discount to its par value, Say ₹97. After 91 days, you will get back ₹100, and therefore you make a return of ₹3.

This is why the allotment value shown on Coin will be lesser than the par value of ₹10,000 (or multiple of 10,000).