Reduce payments now Offset mortgage calculation

fldz

Hi, I'm looking for people with offset mortgages experience that could explain a discrepancy in payment reductions upon offsetting. We have an offset mortgage with Accord and I was trying to work out the reduction in monthly payments after offsetting. It turns out my calculations and any other online calculation platform I have used give a significantly higher payment reduction than what Accord applied.
Here are the details of the mortgage:
Loan: £474,000
Term: 20 years
Initial 27 months Fixed interest rate: 4.79%
Next 36 months interest rate: 6.74% (variable)
Next 177 months interest rate: 7.99% (variable)

We made two payments with no offset, then before month 3 added £14,000 on the offset savings and had the monthly payment recalculated.
My calculations are giving a reduction from £3073 to £2979, by recalculating the interest charged on the loan balance minus the savings from month 3 to the end of the term.
An online calculator (onlinemoneyadvisor) gave me £2983, so close enough to what I get.
Accord came up with a reduction of monthly repayments to £3055 only.
I had them on the phone and two operators confirmed this was correct, without being able to give details about their calculations.

Would anyone with offset mortgage experience be able to explain this discrepancy?
Many thanks.