Because she was the head at a secondary inner-city free school, where a large proportion of the intake achieved good SATs results in Year 6 before arriving at said secondary school.
The reason these kids were achieving and continued to achieve is because they were often from aspirational first- or second-generation immigrant families who prioritised and valued education. Typically, the parents were supportive of the school, wanted their kids to achieve and do well in the UK, and had a culture of discipline at home. It was a kind of selection through the back door. If the children were not on course or predicted to achieve at KS2, Birbalsingh would not admit them to the Free School.
A Freedom of Information request to The Guardian confirms a ludicrous proportion of the intake achieving outstanding results in Year 6 - results which have no equivalence at mainstream schools.
Yes, many of the kids were on free school meals, but poverty vs affluence doesn't always equate in a straightforward way to values and culture, as most right-thinking people understand.
She then proceeded to make a song and dance about her transcendental methods, how she had turned kids' lives around, how her culture of discipline and high-expectations provided children with a quality education - the like of which they would not have had in the local mainstream, non-selective comprehensive (or which they'd have had to pay a fortune for, privately). Remember, these children already have discipline at home and high-expectations at home, which truly makes the difference.
It is easier to achieve results if you have a half-decent intake. Birbalsingh never tried to replicate her success in a predominantly white, coastal school. I'm thinking of Hastings, Norfolk, the Isle of Wight, Blackpool... all of which are reliably at the bottom of league tables. I taught in such a school briefly, and when I phoned home, to inform parents of a detention, a typical response would be 'Good luck with that, if you can get him to go'. If you spoke to a pupil about passing an exam, you'd get 'Well my mum has no qualifications and she is comfortable on the dole'.
Birbalsingh attributed all her pupils' and the school's success to herself, and not to the school's location (ethnically diverse; London) and certainly not to her policy of covert selection.
She is an absolute charlatan. She is also self-aggrandising and dishonest.
Apologies for the long post.