It could be because plant loop is not meeting the setpoint.
From IORef:
Plant Supply Side Cooling Demand Rate [W]
This is the value of the net
demand required to meet the cooling
setpoint of the loop. If the loop
setpoint is met for the current HVAC
timestep, Plant Supply Side Cooling
Demand Rate will equal sum of the
total cooling demand from the demand
side coils on the loop. It will also
equal the cooling output of all
chillers (or other cooling equipment)
on the loop less any pump heat added
to the fluid. For example, for a
chilled water loop with one chiller
and one pump serving one chilled water
coil: Plant Supply Side Cooling Demand
Rate will equal the chiller evaporator
heat transfer less the pump heat to
fluid, and it will also equal the
chilled water coil total cooling
output.
If the plant loop setpoint is
not met, Plant Supply Side Cooling
Demand Rate will equal the sum of the
total cooling demand from the demand
side coils on the loop plus the
additional cooling required to bring
the loop flow to setpoint. If the loop
remains off setpoint for successive
timesteps, the demand required to
return to setpoint will repeat in each
timestep until the loop reaches
setpoint. For this reason, Plant
Supply Side Cooling Demand Rate should
not be summed over time, because it
will overstate the demand whenever the
loop is off setpoint
