Think of compressed air for a moment like electricity: the air pressure is the voltage, and the airflow is the current. If you have a voltage driving current through a resistance (e.g.: a too-thin wire), there will be a loss of voltage (voltage drop) from one end of the wire to the other.
In your multi-compressor system, the pipes leading from each compressor to the common reservoir should be as large as possible, with no narrow points (at valves, connectors etc.), and there should pereferaby be a separate pipe from each compressor to the reservoir (instead of joining them all together at some earlier common junction point)
When air is drawn from the system (by users, etc.), the pressure as measured at the output from each compressor will be slightly higher than that measured at the common reservoir. Hence, for control purposes, it is important that the pressure measurement is made at the right place, so as to most accurately reflect the status of each compressor; but it is probably easier to make one measurement at the common reservoir, which may therefore vary from the exact reading at each compressor.

The more pressure drop there is as a result of narrow pipes etc. between the compressors and the reservoir, the more significant will be the pressure reading error.