Penalty question

Dear Tutor,

SARA RECEIVED NS&I INVESTMENT ACCOUNT INTEREST OF 5,300 IN THE TAX YEAR 2019/20 WHICH SHE DELIBERATELY OMITTED FROM HER SELF-ASSESSMENT TAX RETURN FOR THAT TAX YEAR. SHE DID NOT ATTEMPT TO CONCEAL THE OMISSION.

In this question I understand penalty % but what I am confused is that NS&I investment shoud be exempt. I would be appreciated if you can let me know what I am missing. Thank you
(Why penalty is based on 5,300 amount?)