My referral code is for $40 but my friend says he only received $36?

[Thien11]

Hi there,

On my invite page, it states "Friends who sign up for Airbnb with your link will get $40 off their home booking." I recently had a friend sign up with my code, but he tells me that during the checkout page, it states that it is only $36 off for a 3-night stay in NY worth over $300. He sent me a screenshot of this and I was confused on why he is missing $4? 

Thanks!