Why does the grab function in the vesting contract need two slots to complete?

Question

This is about Week3/Homework1 of the third Plutus-Pioneer-Program.

Why does the grab endpoint need two slots to resolve and release the funds back to the wallet1?

1. This will not create a transaction

2. This will create the transaction

Code to the contract

Answer 1

This happens because 1 slot is not enough to meet the interval when using Constraints.mustValidateIn.

Check this post for more information on why that happens: Constraints.mustValidateIn in a StateMachine