Premium Bond Calculator On Moneysavingexpert - Is there an error?

HUMBUG

The link is below:
https://www.moneysavingexpert.com/savings/premium-bonds-calculator/

When it was comparing winnings to a saving account using 50k at 4.85%  at  Basic Rate Tax it gives a return value £2140. 

But 50,000 x 4.85% = £2425

Therefore 80% of £2425 = £1940 

Is my maths flawed?

Caz3121

You forgot the savings allowance...£1000 for Basic Rate tax payers
https://www.gov.uk/apply-tax-free-interest-on-savings

50,000 x 4.85% = £2425

20% tax payable on £1425 = £285

£2425 - £285 = £2,140