It's not just about rote learning, It is much more complex. Culture, the drivers for migration, standards of living prior to migration, level of their parents education, work ethic, where they live, what investment has happened in schools there and so on.
One of the most underperforming groups is white working class boys in rural areas, white working class boys in Central London who attend a new build academy which has attracted lots of investment and is surrounded by people living an aspirational lifestyle may well do better than those boys who live in an isolated rural area and aspire to work on a farm and therefore see no point in gcse's.
Indian children who move here are likely to have parents with skills and education already, with a strong motivation to be successful. The same is true of white kids. What the data isn't showing you is a true reflection, there are millions of children in India living in poverty being raised by parents who didn't go to school, who don't read with their kids or send them to tutors who won't have good gcse outcomes, but they typically aren't the ones who migrate here on account of having no money or skills.
Again for other ethic groups like black children, we can't measure the impact of decades of racism in communities, for example how trusting will parents be of authority when you look at things like the Windrush Scandal. This has a complicated impact on culture and aspiration, for example some bright young black boys in London are attracted to gang culture in a way that a bright young Chinese boy wouldn't be.
The government does identify under performing groups and schools get additional money to support them. The bigger issue is of course that education is woefully underfunded and only those whose parents can afford to top it up with tutoring, swimming, sport and music lessons. Or who can afford to move to a more affluent area with a grammar school or state school populated with children from similar backgrounds will have get the best outcomes.