National insurance calculation

So I was looking at my tax and NI contributions on the app, what I dont understand is when I put my amount earned in the tax calculator it comes up that I should be paying £850 less in NI that I have the year just gone, I don't pretend to know much about it and the calculator says its an estimate but this seems a large difference could you help me understand this?

Hi,
If you contact the National Insurance helpline or use web chat services via a personal tax account, they can look at the amount you have entered in the app
and check the details held on your account.
They will check the amounts for year that has finalised against the details you provide for your employment as it is based on the frequently you are paid and the amount.
Number - 0300 200 3500
Overseas - 44 191 203 7010
Open - Monday to Friday: 8am to 6pm Closed on Saturdays, Sundays and Bank Holidays.
Thank you.