Yes it does seem strange and the possibilties are almost limitless. It could be the servos that did not come with the radio are more sensitive to the information on the servo's command wire (typically the white wire on one side as power and ground are center + and side -). It could be that some signal is ending up on the power lead. It could be that what is called the "local oscillator" inside the receiver is somehow getting through to the command wires. It's one of those troubleshooting trial and error events when you do not have an oscilloscope to actually "see" what is what. All you can do is look for what is the most likely cause. Over the last 50 years I have seen many strange things and designs that are far from "text book". Had one voltage regulator (BEC) in a piece of military equipment where a capacitor was left out of the circuit and under certain conditions this BEC became like a AM transmitter causing all kinds of strange activities! In television we have often sent corrections to simple ciruits from big companies. My goal is to assit you by walking you through some of the basic steps of troubleshooting to see if we can isolate the cause. Knowing that you have the cleanest ample power possible is the first step for without this narrowing down the root cause could be difficult to determine. As for other servos aside from the one with that came with the radio, if they are from the same manufacturer you could have obtains some defects. For example, if they forgot to add the "bypass" capacitors across the servo motors, when the motor start to turn it will produce a spark, the servo will think this is a command, and start to move, creating another spark, and the result is chatter. This is probably not the case and you get the idea as to what we are up against. If you wish, you can send me one of your worst chattering servos and I can try it for you on my receiver.
Just saw your post about AM and PM and yes there is a very big difference! AM is "Amplitude Modulation" where the receiver interperts changes in the power of the transmitter. PM is "Pulse Modulation" where "Pulses" are sent. Modulation means how the basic signal (72 mhz) is changed. We typically call AM "Ancient Modulation"
AM varies the power, FM varies the Frequency (such as varing from 72 mhz to 73 mhz for understand purposes only).
There should be no voltage change during operations. If the voltage drops you do not have enough current mAh available.
I truly hope all this has value to you and anyone else who reads our exchanges.