Here is some explanation:
According to Abba: The Official Photo Book, published to mark 40 years since they won Eurovision with Waterloo, the band’s style was influenced in part by laws that allowed the cost of outfits to be deducted against tax – so long as the costumes were so outrageous they could not possibly be worn on the street.
In 2007 Ulvaeus was wrongly accused of failing to pay 85m kronor (£7.9m) in Swedish taxes between 1999 and 2005, and went on to successfully appeal against the decision.
“I am of course very happy that I have been informed in writing that I have always done the right thing concerning my taxes,” he said after the court victory.
The article is here, via Ted Gioia.