It's whatever suits them on the night. I thought the stated aim of the dance off was to guard against 'popular' dancers being eliminated at the expense of those who are technically 'better' because 'it's a dance competition'.
Personally, I couldn't have cared less about tonight's result. I think Mark has reached his limit in terms of improvement trajectory and Ben could have done better if he'd been willing to work harder, but wasn't.
After Mark leaves next week the competition will become closer.