More incorrect than relevant - I suspect you parsed the wrong line.

I'm seeing:

   At the end of the first quarter of 2024, there were 87,869 prisoners in England and Wales.

and:

  76,869 British nationals were recorded in the prison population at the end of the first quarter of 2024. The remainder of the population was made up of 10,422 foreign nationals, making up 11.9% of the total population

The math checks out: 10,422 / 87,869 x 100 = 11.86%