The package is awarded by LTA to SBST to run the services, and
use the leased LTA owned buses to run them. They cannot anyhow just
swap their own fleet into the package to use. And SBST has its
other services to run. They won't have enough buses if they take
away and swap into the package.
Remember everything now is owned by LTA. The bus operators are
just service providers and they have not much decision making on
the fleet and packages.
If you look at the deployment pattern of Amdep for their Yishun
svc,Many SG Citaro always perm on some Yis Feeders,851,852 &
860.But i do have a feeling that SBST might just take the SG
Citaro,SG A22 & A95 and not Enviro500.