This is due to the VAT figures not being rounded up or down properly.
Assuming the pre-VAT figures are �135.58 and �566.39 the result of your calculation would yield �159.30650 and �665.50825. These are shown as rounded to the nearest 2nd decimal in the column but are actually treated as the 5 decimal place figure for the sum calculation i.e. �824.81475 which is rounded to the nearest 2nd decimal.
To counteract this behaviour change your formula from
=A1*1.175
to
=ROUND(A1*1.175,2)