The Swedish and Danish flags are older than the very idea of standardisation. Finland is the thousand year subject of Sweden and took its form. Norway and Iceland are thousand year subjects of Denmark, and took their form. Finland, Norway and Iceland all received their independence on good terms and simply kept the flag as is.

The miniscule differences between the Swedish/Finnish ones are simply explained with that Sweden, with being the compulsive administrator that it is, fine tuned its proportions when the law about the flag was changed/clarified while Finland just kinda didn't care. Norway and Iceland got their independence late and the idea of math and vexillology was a much more relevant matter.