anniebach Essex local authority has mainly comprehensive schools, apart from four grammar schools, which are super-selective. They take about 2-3% of the most able pupils.
Southend, although geographically in Essex, also has four grammar schools, which are much less selective, because it's a much smaller authority.
Essex parents try to get their children into the Essex grammar schools as a first option, then they try for one of the Southend grammar schools. Approximately 40% of pupils in Southend grammar schools come from outside Southend, mainly Essex. This affects the ability of the Essex comprehensives, because some of the most able have been "creamed off", although Essex is a mainly comprehensive authority.
The opposite happens in Southend. Parents try to get their children into the grammar schools, but if they fail, the parents then try for one of the Essex comprehensives, which achieve higher results on average than the Southend non-grammar schools.
The result is that Southend non-grammar schools don't have the (approximately) 25% most able. They also lose those children who have been able to get into Essex schools. Not surprisingly, the Southend non-grammar schools achieve low results, because their intake mainly consists of the least able. Of course, parents of able children in Southend don't want them to go to schools where most of the intake is of below average ability and course offered are generally "vocational", so they have little choice but to try for grammar schools or apply to an Essex comprehensive as first choice. It's just tough luck if a child has a cold or family issues on the day of the 11+ and their whole life and future is affected.