My guess is would allows for a bit more slack in the 450/444 availability. I presume that currently the Desiro fleets are being stretched as much as possible to cover the shortfall in 442s ?.
I would imagine the Desiro fleet is now pretty relaxed, there’s now a built in spare that sits in Waterloo platform 19. It comes in as 2P98 during morning peak and sits there until 1600ish where it forms another service. The main issue lately is crew, where desiro stock in the evening peak has been split and made 5 v 10 to run the services, as there isn’t enough 442 trained guards, but there’s not enough in general to release those for training.