You are half right, however it varies from operator to operator.
CDG and BT & Tan has a fixed set of deployment, and BT &
Tan has spare Golden Dragon coaches (which I haven't seen on
service yet).
Tong Tar also has spare Zhong Tong Sunnys.
How sure are you with BT&Tan's deployment when the
(supposedingly) 912P buses appeared on 268P, how sure are you that
these are redeployments and not just a one day deployment, since it
was just the first day of operations for 268P.
It really depends on where the buses are parked. ComfortDelgro
buses are usually fixed to a driver, who would usually park it at a
HVP near their homes, this means the same driver will drive it
everyday except on off days, which explains the "fixed" deployments
you see. BT&Tan and Tong Tar on the other hand have most of
their buses parked in one or two locations (some may be parked at
HVPs due to space constraints), meaning that drivers can be asked
to drive any suitable bus available.