Assumiing that isn't normal for the module, the chances are they are AM. When a '1' is transmitted, a carrier is sent and the receiver presents '1' at the output because it is receiving it. When '0' is transmitted, the carrier is actually turned off and the receiver sees no signal, only random noise. If you are using a voltmeter to read the votage, the chances are the random noise reads as something betwen 0V and 5V, depending on other interference sources nearby.
Brian.