Stupendous
To answer your question... because an SSR code is not a value of hundreds and thousands, but is a set of four code digits. E.g. I am sure that you would not refer to an aircraft with radio failure as "squawking seven thousand, six hundred"?
ATCO Two
My point is that if you think that the "operational" habit is more pertinent, put up a case to SRG, get CAP413 changed, and the whole subject will be sorted out formally. Otherwise, the colleges remain obliged to teach precisely what is intended by both ICAO and the UK CAA. If they do not adopt this principle, then that is the start of a very slippery slope.