Michael Rice's bottom-placed song, Bigger Than Us, had picked up 16 points in Saturday's bash in Tel Aviv.
But organisers have now said that was too optimistic - by five points, and so 11 is the revised total.
The Eurovision Broadcasting Union (EBU) said an incorrect calculation was used to create a substitute score after the Belarussian jury was dismissed, resulting in the error.
Belarus's jury was removed after its semi-final votes became public, breaking contest rules.
The EBU said it "deeply regrets" the mistake and has blamed "human error".
In other changes, Dutch winner Duncan Laurence's song, Arcade, was awarded another six points, bringing it up to 498 points.
Norway dropped to sixth from fifth after having points deducted.
The UK's 26th place finish has been seen by some commentators as a Brexit-related snub.
It was the first time the UK had come last since 2010.
The Netherlands were the bookies' favourites, winning for the first time in 44 years.
Madonna performed during the show, singing her 1989 hit Like A Prayer with a 35-strong choir and her new single Future, alongside American rapper Quavo.
During the performance, her dancers turned their backs to the crowd and were revealed to have Israeli and Palestinian flags on their backs.
They linked arms and walked together in the same direction moments before the words "WAKE UP" appeared on a screen behind Madonna and Quavo.
Organisers said it had not been part of the approved act and that the star had been "made aware" the event is "non-political".
© Sky News 2019