Hmmm,
I'm assuming that both engines are running and the isolation valve is closed. If the engines are at idle then I think it could either be a leak as PT states or quite simply that the packs are supplying a different demand.
I'm not sure which variant you are refering to (-300/400/500) but the left pack supplies in the main the flight deck direct and recycled left and right supplies the cabin (fresh air for the drivers the rest get the dregs
)
As the cockpit is a smaller area demand temperature is reached a lot quicker so, assuming hot OAT, for the left pack the mix valve will be partly open (some bleed air used to operate the air cycle machine AND some bypassing to achieve an output temp close to the desired temp). The right pack however is more likely to have its' mix valve closed and the ACM working harder as the combination of a high OAT, large area and lots of heat producing passengers. This air path is likely to produce more resistance and thus less mass flow.
Therefore it is possible that the left duct pressure will be lower than the right due to the different flow rates through the packs.
Conversely when operating when OAT is very cold the opposite may be the case.
If you get a little time on the ground play with the temp control knob for each pack and watch the duct pressure rise and fall as pack demand varies.
I have operated packs on the ground with OAT at -25 and have been lucky to get 10 psi duct pressure from the APU and not much more from the engines!! Shut the packs off however and it will climb to 30 psi.
This may be what you are seeing. It could be a load of
of course.
