Putting the cart before the cause?

New research has shown that, in primary schools, staff with a good work-life balance are more likely to produce higher achieving pupils – defined as those attaining level four or higher in Key Stage 2 level Maths and English.

Or does it show that staff who teach pupils who attain level four or higher in Key Stage 2 level Maths and English have to spend less time at work – i.e. helping pupils who struggle to attain said levels?