Quite possibly because that is what the CAA says at the bottom of the PDF
"(6) In order to preserve market analysis all identifiable diversions are reallocated to the point of intended operation."
That applies to their route analysis in Table 12.1.
But does it also apply to their overall totals as shown in Table 9? Don't see a qualifying note there.
https://www.caa.co.uk/docs/80/airpor...ransit_Pax.pdf