Defined Benefit Discount rate

Dear Sir,

I was reading the annual report of sainsburys on there defined benefit plan and read

“The £589 million decrease in the deficit from
11 March 2017 was driven by a rise in the discount rate from 2.70 per cent to
2.80 per cent, and updates to future mortality assumptions. The discount
rate has been increased this year to use a revised approach that the Group
believes better reflects expected yields on high quality corporate bonds
over the duration of the Group’s pension schemes.”

How would an increase in discount rate decrease a deficit? isn’t both Assets and liabilities are both affected by the same discount rate so the net effect would be zero?