Further to previous answers, showing the results show on a Sunday night allows the BBC to edit the 2nd part of SCD into a 45min programme.
With the professional dancers appearing sometimes up to 3 times on the results programme, extra time is needed for costume changes etc.
The BBC have explained their decision along the lines of the results show, if broadcast on a Saturday night, would not be over until after 9pm, which is too late for wee ones to be up and the early evening showing on a Sunday allows younger children to see both the live show and the results as well.
Personally I am surprised the results have not been leaked to the media, unless they have a code of agreement not to disclose them in return for access to the performers perhaps.